"Science, Faculty of"@en . "Physics and Astronomy, Department of"@en . "DSpace"@en . "UBCV"@en . "Yarker, David Richard."@en . "2009-02-09T22:17:32Z"@en . "1995"@en . "Master of Applied Science - MASc"@en . "University of British Columbia"@en . "We have fabricated separate confinement heterostructure, graded index, indium gallium\r\narsenide (InGaAs), strained layer, quantum well, ridge waveguide lasers. This thesis\r\ndescribes both the fabrication procedure we have used and the results of our characterisation of these devices. Our fabrication procedures differs from other common procedures in that we have used carbon for the p-type dopant and e-beam lithography to define the ridges.\r\nCarbon doping has been selected rather than beryllium because carbon does not diffuse\r\nrapidly in GaAs during the growth of the laser. Electron beam lithography has several\r\nadvantages over conventional lithography including finer line widths and greater flexibility in pattern definition and pattern layout. We have demonstrated that ridge waveguides can be defined by electron beam lithography, and we have obtained good ridge profiles, etch resistance and write times. Characterisation of these devices used both electrical and optical techniques. DC and pulsed current-voltage characteristics have been measured for devices for different\r\nsubstrates. Substrate materials included two samples with doping pulled away from the\r\nquantum well, one doped with beryllium and the other with carbon, and one sample with\r\nthe doping close to the quantum well doped with carbon. Diode ideality factors were\r\ndetermined and correlated to the doping profiles using a novel two diode model for the active region. In this model the active region is modelled as two separate diodes in series, separated by the quantum well, which acts as the source for the minority carriers.\r\nThe series and shunt resistance were also determined from the current-voltage\r\ncharacteristics. Typical values for series resistance were about 10\u00CE\u00A9. Shunt resistances\r\ncover a wide range of values from 100 k\u00CE\u00A9 to greater than 500 M\u00CE\u00A9. We have also\r\nmeasured the specific contact resistance of an indium-silver n-type ohmic contact to be no more than 1.9 10\u00E2\u0081\u00BB\u00E2\u0081\u00B4 \u00CE\u00A9cm2 using a coplanar dot resistance method described in this work. We have also measured the specific contact resistance of a chrome-gold p-type ohmic\r\ncontact to be (12\u00C2\u00B15) 10\u00E2\u0081\u00BB\u00E2\u0081\u00B5 \u00CE\u00A9cm\u00C2\u00B2 using a series resistance measurement.\r\nThreshold current densities of about 2.0 kA/cm\u00C2\u00B2 have been obtained for ridge\r\nwaveguide lasers about 500 \u00CE\u00BCm long and 3 \u00CE\u00BCm wide on Be doped laser substrates. A\r\nsimilar laser was found to have a carrier lifetime at threshold of 11 ns from turn-on delay\r\nmeasurements. The value of T\u00E2\u0082\u0080 for a laser 240 \u00CE\u00BCm long and 6 \u00CE\u00BCm wide has been measured\r\nto be 88 K below 10\u00C2\u00B0C and 28 K above that temperature. This laser is also shown to lase\r\nin the second quantum subband at sufficiently high drive currents. Electroluminescence\r\nspectra have also been collected and compared to theoretical calculations."@en . "https://circle.library.ubc.ca/rest/handle/2429/4306?expand=metadata"@en . "7449793 bytes"@en . "application/pdf"@en . "F A B R I C A T I O N A N D C H A R A C T E R I S A T I O N O F R I D G E W A V E G U I D E InGaAs Q U A N T U M W E L L L A S E R S by DAVID RICHARD YARKER B.Eng. Royal Military College of Canada, 1992 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE F A C U L T Y OF GRADUATE STUDIES DEPARTMENT OF ENGINEERING PHYSICS We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August 1995 \u00C2\u00A9 David Richard Yarker, 1995 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of PLy 14>15>16. The work function of a metal (tym) is the energy required to remove an electron from the surface to the vacuum. This is the energy required to raise an electron from the Fermi level to the vacuum level. Since the Fermi level of a semiconductor is in the band gap and is dependent on the doping level of the semiconductor, the work function varies from crystal to crystal. A more useful energy for semiconductors is the electron affinity (%s), which is the energy required to remove an electron from the bottom of the conduction band to infinity. This is a constant for a given crystal. Figures 3.1a through d show the formation of a Schottky barrier in a metal-semiconductor contact. 35 Figure 3.1a: Metal and semiconductor far from each other Figure 3. la shows a metal on the left and a semiconductor on the right. In figures 3.la through 3.Id 6c is the conduction band energy, e v is the valence band energy and ef is the Fermi level. Figure 3.1a clearly demonstrates the difference between <|)s and %s. K < 1 r4> e f e V Figure 3.1b: Metal and semiconductor in electrical contact but far apart. When the metal and semiconductor are placed in electrical contact then the Fermi levels will be equal. This requires a shift in the vacuum levels, which represents a potential difference between the two equal to the difference in the work functions (m - s-This idealised analysis does not really apply to GaAs (or AlGaAs for that matter) since the surface states of the crystal have been ignored. The electronic structure of the surface of GaAs is not identical to the bulk because of surface states, surface damage and surface 37 contamination. These effects create electronic states in the gap of the semiconductor at the surface. These states dominate the behaviour of contacts to GaAs 1 4 . The states in the gap may be charged due to charge exchange with the bulk, which will create a surface space charge layer. In the case that the surface states extract electrons from the bulk, band bending similar to that shown in figure 3.2 is obtained. b Surface States \u00E2\u0080\u0094 e V Figure 3.2: Band bending in the presence of surface states In this case the band bending is insensitive to the metal contact layer. If a metal is brought into contact with the semiconductor, and there are sufficient surface states to support the field between the metal and the semiconductor, then there will be no additional band bending. In this case the barrier height is independent of the work function of the contact metal and is dependent only on the electronic structure of the semiconductor surface. This phenomena is known as Fermi level pinning. In GaAs surface states tend to pin the Fermi level at a position two thirds of the way from the valence band to the conduction band. 1 7 Fermi level pinning is discussed in more detail elsewhere 1 7' 1 8' 1 9. Since potential barriers are the rule rather than the exception in metal/GaAs contacts I will now outline how current flows through these barriers. There are three mechanisms for current flow past potential barriers: thermionic emission, field emission and thermionic field emission.20 These conduction mechanisms are schematically represented in figures 38 3.3 Thermionic emission is current conduction by thermal excitation of the carriers over the barrier. Thermionic emission is dominant for barriers with wide depletion regions. Figure 3.3a: Thermionic emission Figure 3.3b: Field emission Figure 3.3c: Thermionic field emission Field emission is quantum mechanical tunneling through the barrier and dominates in situations where the barrier is narrow. Thermionic field (or thermally assisted field) emission is a combination of the two. In thermionic field emission carriers are thermally excited part way up the barrier to where tunneling is probable. In most practical ohmic contacts thermionic field emission dominates21. It has been shown that in the case of thermionic field emission the resistance is given by 2 2 : 39 (3.1) Where b is the barrier height, Nd is the doping concentration in the semiconductor, k is Boltzmann's constant, T is the temperature, q is the electronic charge, h is Plank's constant divided by 2k, m* is the effective mass of the electron, e is the dielectric constant of the semiconductor and Rc is the contact resistance. Equation 3.1 suggests two possibilities for making low resistance ohmic contacts. The first is to minimise the barrier height (<|>b). The other choice is to maximise the expression in the denominator of the exponential in equation 3.1. Everything in the denominator is a constant for a given material except the doping concentration, N Pulse Generator Figure 4.3: Pulse current measurement circuit The voltage drop across the diode is the difference between V totai and V r and the current through the diode is determined from the drop across Ri. The uncertainty in the bias is large (>10%) since the bias is the difference between two numbers of the same order of magnitude, each with an uncertainty of about 5%. To ensure that there were no light induced effects the samples were tested in the dark. Where possible the temperature was also monitored. For each laser both DC and pulsed I-V data was taken because the DC technique is limited by drift in the bias at high drive currents. This drift may be caused by thermal heating at the bonds or the contacts. The temperature, as measured by the thermocouple attached to the laser block, increased by 47 more than 10\u00C2\u00B0C at high DC drive currents. The temperature increase in the junction may be much higher. At high biases the current through a diode is limited by the series resistance. In this regime the current ideally varies linearly with applied bias. In our case series resistances were always determined using the pulse technique. The series resistance is determined from the slope of the high bias linear region of the I-V curve. The curves for our lasers were not always linear at high bias. However, since in all cases we fit a straight line to a curve that is either linear or superlinear, we can only over-estimate the series resistance using this method. Bias (V) Figure 4.4 Typical series resistance of 15 Q The pulse data is noisy due to the uncertainty in the measurement technique, as can be seen in figure 4.4. A typical fit, in this case for the NRC material run #30 laser #36, is presented in figure 4.4. Table 4.1 is a representative list of the series resistances measured 48 in this work. Typical values in the literature cover a wide range, but values around 6Q are typical for ridge waveguide lasers.11 Material Laser Rseries (Q) Material Laser Rseries (Q) NRC #30 6 7.1 423 #34 5 3.6 30 9.1 15 13 36 15 423 #42 13 14 37 13 421 #38 2 11 38 12 10 19 NRC #39a 41 6.7 21 ' 7.1 43 6.3 NRC #39b 30 8.5 32 4.5 Uncertainty -10% Table 4.1: Series Resistance The shunt resistance is also obtainable directly from the I-V data for a diode. It is given by the slope of the linear portion of the curve at low bias. In the cases where the shunt resistance was so high that there was no linear portion, a line was fit to the two lowest bias points to provide a lower limit on the shunt resistance. Where a lower limit is implied it is noted in the text. Table 4.2 gives typical shunt resistances measured for this work. Shunt resistances are hard to find in the literature, however measurements on a semiconductor laser fabricated by BNR gave a value of 2.5 MQ,. 49 10- 2 n\u00E2\u0080\u00941 \u00E2\u0080\u0094 1 i 1 1 T\u00E2\u0080\u0094| 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 [ 1 1\u00E2\u0080\u0094t~i 10\" 3 -i O : 10- 4 O < o -\u00E2\u0080\u00A24\u00E2\u0080\u0094' f\u00E2\u0080\u0094 10\"5 o CD $ \u00E2\u0080\u00A2 i_ i_ 10\" 6 r \u00E2\u0080\u0094 o f FW9-6 M Q \u00E2\u0080\u00A2 10\" 7 1 10- 8 -: 10- 9 i . . . 1 . . i . . . i . . . i . . . i . -0 0.2 0.4 0.6 0.8 1 1.2 1.4 Bias (V) Figure 4.5: Typical shunt resistance measurement of 9.6 MQ. Material Laser Rshunt ( & ) Material Laser Rshunt ( Q ) NRC #30 6 6.8 107 423 #34 5 8.8 105 30 5.0 108 15 * 36 4.9 108 @ 423 #42 13 1.0 105 37 4.8 108 421 #38 2 9.8 106 38 2.0 108 10 9.2 106 NRC #39a 41 1.0 105 21 8.8 106 43 1.6 105 Uncertainty -20% NRC #39b 30 1.1 106 @ this point represents a lower limit * Data not acquired for this device 32 2.8 105 Table 4.2: Shunt resistance 50 The diode ideality factor, n and the reverse saturation current, Io for a diode are both calculated by fitting the exponential region of the I-V curve. The exponential region is taken as the linear portion on a semilog plot. The current in this region is assumed to be given by equation 4.1 so both n and Io are obtained from the fit. A typical curve, laser #36 from NRC #30, is shown in figure 4.6 along with the best fit line, which gives values of n=2.4 and Io=1.2 10 - 1 1. The diode ideality factor here is greater than two, which does not agree with the results of standard analyses in the literature23'32. These theories, and an explanation of our results are presented in section 4.2. Table 4.3 includes a listing of the values of n and Io for typical laser structures fabricated in this work. Figure 4.6: Typical n and IQ determination 51 Material Laser n Io (amp) Material Laser n Io (amp) NRC #30 6 2.2 3.0 I O \" 1 1 423 #34 5 3.0 5.5 I O \" 8 30 2.2 5.0 I O \" 1 3 15 3.7 2.7 I O ' 7 36 2.4 1.2 I O - 1 1 423 #42 13 2.6 3.0 I O - 7 37 2.4 1.2 I O \" 1 1 421 #38 2 1.8 5.6 I O \" 1 2 38 2.7 1.5 I O \" 1 1 10 1.5 3.7 10-12 NRC #39a 41 2.2 2.5 10-7 21 1.6 5.9 I O \" 1 1 43 2.2 1.4 I O \" 7 NRC #39b 30 2.2 1.8 I O \" 8 Uncertainty in n ~ 15% 32 2.7 1.7 I O \" 7 IQ accurate to order of magnitude only Table 4.3: Diode factors 4.2 Discussion This section outlines the theory of electrical conduction in p-n junction diodes and then evaluates our laser design and fabrication procedures. The theory includes a discussion of the Shockley equation and the Shockley Read Hall recombination model. To evaluate our lasers we first determine the degree to which the current is confined to the ridge. This involves a determination of the current flowing through the isolation layers surrounding the ridge structures. Next, the contact resistance of the p-type ohmic contact is determined from the series resistance measurements. Finally a mechanism for diode ideality factors greater than two is presented. 4.2.1 Electrical Conduction in P-N Junction Diodes A semiconductor diode laser is a p-n junction designed to cause radiative recombination in the depletion region to create light. Electrically then a semiconductor laser is a p-n 52 junction diode. The theory of conduction in p-n diodes is discussed in many texts 2 3' 3 2, so I will only briefly summarise the essential results from these texts. The current through a diode due to the recombination of minority carriers in the neutral regions is given by the Shockley equation:33 J = J0 ( i t \ ,kT _ J (4.2) where J is the current density, V is the diode bias, q is the electronic charge, k is Boltzmann's constant, T is the temperature, D n and D p are the electron and hole diffusion coefficients, L n and L p are the electron and hole diffusion lengths, and pno and npo are the minority carrier concentrations on either side of the junction. The Shockley equation assumes no recombination or generation in the depletion region. For semiconductor laser diodes this assumption may be poor because laser diodes are designed to maximise radiative recombination in the depletion region. The standard method for describing non-radiative recombination is the Shockley-Read-Hall (SRH) model. This model calculates the recombination rate for a state in the gap of a semiconductor. This rate is maximised when the state is at the centre of the gap. The current due to recombination in the depletion region is obtained by integrating this recombination rate over the depletion region to obtain:33 Jrec \u00E2\u0080\u0094 _ c 1 (4.3) where q is the electron charge, ni the intrinsic carrier concentration, W is the depletion width, x is the carrier lifetime, and V is the bias across the diode. Under forward bias both equations 4.3 and 4.2 have the form of an exponential in the applied bias multiplied by a constant. Equation 4.3 differs from 4.2 in that the exponential term involves V/2 and not V and the constant is different. The total current through the diode can be approximated by the sum of the diffusion current, which is given by the Shockley equation, and the 53 recombination current, given by equation 4.23 3. Empirically, as mentioned in section 4.1, conduction through a diode at high bias is given by: /o=/ 0 | enkT -1 (4.1) where n is the diode ideality factor. A comparison of equations 4.1 through 4.3 shows that if n=l then diffusion current dominates and if n=2 then recombination current dominates. In cases where n is between 1 and 2 the current is generally taken to be a combination of the two current mechanisms. In this regime a straight addition of the two currents is no longer valid. Diode ideality factors are discussed in more detail in section 4.2.3. 4.2.2 Electrical Isolation Two methods of electrical isolation have been used in this work, alumina layers and high resistance contacts to AlGaAs. In an attempt to determine the effectiveness of these isolation techniques I-V curves for Cr/Au contacts to alumina layers and AlGaAs layers without ridge waveguides have been measured. First I will discuss the results of our measurements on the AlGaAs layers and compare our results with those in the literature. Following this I will compare the results with AI2O3 to the results on AlGaAs. Amann 2 4 reports producing lasers using Cr/Au deposited directly on AlGaAs where, at threshold, 99.94% of the current flows through the ridge and the current density under the ridge was 5 104 times greater than that in the surrounding regions. We have made similar measurements on our own structures to ensure that we are getting sufficient isolation. First I will examine the conduction an AlGaAs contact on the NRC material. Figure 4.7 shows the total current flowing through the ridge and the surrounding AlGaAs, and the current flowing through just the AlGaAs region. The laser curve was measured directly from laser #36 on the NRC material. The AlGaAs isolation curve was measured on a separate piece of material with no ridge waveguides, and then scaled to the appropriate area for the 54 AlGaAs surrounding laser #36. Table 4.4 shows the percentage of the current flowing through the AlGaAs at various bias levels. Bias (V) Figure 4.7: A plot of the current flowing through the ridge waveguide structure and the surrounding AlGaAs, on the left, and the current flowing through just an equivalent AlGaAs area on the right. Bias 1.5V 2V 2.5V 3V 4V Current 18% 14% 11% 10% 10% Table 4.4: Current through the isolation layer for NRC material These results appear to be much worse than those quoted by Amann 2 4 , where only 0.04% flowed through the AlGaAs at 2V. A closer analysis of the two experiments however, shows that the current density in the AlGaAs regions are similar in both Amman's work and ours. From an analysis of the data in his paper we have calculated the current density in his AlGaAs layers for several biases. Table 4.5 compares the results 55 from Amann's paper with results we have obtained on two separate samples, one from material 423 and the other from NRC material. Current Density (A/cm2) Bias (V) Am arm NRC 423 2 0.13 0.12 1 2.5 0.63 0.24 16 3 2.0 0.45 19 3.3 4.0 0.78 22 Table 4.5: Current density in the isolation layer Here we see that the isolation layers on the NRC material are perhaps even less conductive than those quoted by Amann 2 4. This implies that the isolation layer is functioning at least as well in our case as his. There are two reasons why the isolation is as good, and yet the current flowing through the AlGaAs is much higher. The first is that Amann's laser is approximately 10 times more conductive than ours at higher biases. While the series resistances of the structures are almost identical (laser #36 RSeries=14.7\u00C2\u00A32, Amann Rseries=14.6Q) the onset of linear conduction is at much higher bias for our structures. This is discussed in more detail in section 4.3.3. The other difference in the structures is that the AlGaAs region surrounding the laser is 25 times larger in our structure. These two factors lead to 250 times more current flowing though our AlGaAs than through Amman's. The other observation in Table 4.5 is that Material 423 is more conductive than the NRC material. This is probably due to the differences in the dopant concentration and profile in the AlGaAs in the two samples. The NRC material is doped at about 5 10 1 7 while 423 is doped at 2 10 1 8, which should reduce the contact resistance of the Cr/Au contact to AlGaAs. 56 I-V curves were also taken for a sample of 423 with both alumina isolation layers and AlGaAs isolation. Figure 4.8 is a plot of the current flowing through these layers (on the right) and through a laser on sample 423 (on the left). Bias (V) Figure 4.8: Isolation layers on material 423. The curve labeled laser includes the current flowing through the laser and the surrounding AlGaAs. Curves labeled alumina and AlGaAs correspond to I-V's through equivalent areas of AlGaAs either covered in alumina or bare respectively. Figure 4.8 demonstrates the surprising result that the alumina layers appear to provide no additional isolation over the AlGaAs layers alone. In fact, for the curve above, the alumina sample actually conducted better than the bare AlGaAs. The uncertainty of at least 10% on each axis in the above curves leads us to conclude that the alumina has no additional effect on the isolation at high biases (above about 1.5 V). Alumina is an excellent insulator, so this result is unexpected. We are not able to offer a satisfactory explanation for the similarity between the AlGaAs and alumina isolation techniques, but it seems unlikely that the alumina does not act as a barrier to the current flow. It is possible in our structures that the leakage current, here 57 ascribed to the flow through the isolation layers, is actually due to defect conduction at the surfaces or through pin holes. We have not attempted to measure these effects directly. In order to determine if our isolation is acceptable more work is required to determine the amount of current leakage by edge conduction. This type of experiment is possible using the lithographic techniques already developed. If, however, the edge conduction is already minimal then more work is required to improve the isolation of our devices. One method to improve isolation would be to continue to deposit the Cr/Au contacts directly on the AlGaAs but reduce the area of the AlGaAs contacted either by placing the lasers closer together or by using an additional lithography step to define windows around the lasers to put small contact pads on. 4.2.3 Contact Resistance Each layer of the laser structure will contribute a resistance, as will the contacts, the wire bonds, the silver epoxy used to bond the laser, the contacting pins and the grounding wire. The series resistance of the NRC substrate is about 1 Q 3 4 according to the manufacturer. A specified resistance value of 1Q, will also be taken for 421 and 423. The resistance due to the n-type contact is about .05 Q, as calculated using the contact resistance determined in section 3.3.4. The resistance of the current path from the header post connection to the p-type contact has been measured to be about 0.2 Q. Finally the resistance of the current path from the n-type contact to the header post has been measured to be about 0.5 Q. Al l of these factors contribute a total of about 1.8 Q. The remainder of the series resistance is assumed to be due to the p-type ohmic contact. The contact resistance is dependent on the area of the contact so the specific contact resistance (Rc), the contact resistance times the contact area, is generally quoted instead. The specific contact resistances have been determined from the data presented in Table 4.1 and the area of the structures. The contact resistance is calculated by removing 1.8 Q from the values in table 4.1 and correcting for the fact that only 85% of the current flows through 58 the ridge; 15% conduction through the AlGaAs is taken as typical for all structures at high bias. The area used in each case is the ridge area. The results of these calculations are included in Table 4.6. Materia] Laser R c flcm2 Material Laser R c Clem2 NRC #30 6 1.1 10-4 423 #34 5 6.2 lO\"5 30 1.2 10-4 15 1.4 IO\"4 36 1.9 10\"4 423 #42 13 1.0 io- 4 37 1.7 lO\"4 NRC #39b 30 8.6 IO\"5 38 1.6 10-4 32 9.3 10-5 NRC #39a 41 6.9 lO\"5 43 1.6 lO\"4 Table 4.6: Contact Resistance From table 4.6 the series resistance of this contact is taken to be (12\u00C2\u00B15) 10 - 5 Q cm 2 , which agrees well with the values of 7.3 10 - 4 to 4.3 IO - 5 Q. cm 2 quoted in the literature29. While this value is acceptable, lower contact resistances are always desirable. Furthermore some of our diodes, like the NRC sample discussed in section 4.2.2, seem to be highly resistive, even though the linear slope gives a small series resistance. What this means is that, while the calculated series resistance is low, the bias required to reach the series resistance limited regime is quite high, perhaps 4V. There are two possible reasons for this. The first would be that our value of Io is small. In that case the on-set of linear resistance would be at higher biases. An examination of our data however suggests a different mechanism. Our diodes appear to have series resistance limited behaviour at moderate biases, perhaps IV, but the resistance is non-linear, only approaching linearity at higher biases, around 4V. The mechanism behind this behaviour is unknown, but the contacting structures are suspected. 59 4.2.4 Diode Ideality and Recombination In a p-n junction diode, diode ideality factors of 1 and 2 correspond to recombination outside and inside the depletion region respectively. For intermediate values of n recombination occurs in both locations31. We have used this idea as the basis for explaining the diode ideality factors encountered in this work. Io is also related to recombination and the carrier lifetime by equations 4.1,4.2 and 4.3. An examination of the diode ideality and Io values can provide information about the junction region of the laser. Table 4.7 summarises the values for diode ideality and Io obtained for the various materials used in this work. Material n I 0 (amp) NRC #30 2.4\u00C2\u00B10.3 io- 1 1 NRC #39 2.4\u00C2\u00B10.3 10-7 423 #34 3.3\u00C2\u00B10.6 10-7 423 #42 2.6 10-7 421 #38 1.6+0.2 io- 1 2 Table 4.7: Diode factors The diode ideality factors in table 4.7 for NRC and 423 are greater than two, which is not consistent with the diode theory discussed in section 4.2.1. Also note that Io appears to be dependent on the processing technique. In fact Io can vary by four orders of magnitude for the same material. The fact that Io is dependent on the fabrication procedure is not surprising since additional recombination centres introduced by diffusion or physical damage to the sample would lead to an increased Io. Since samples NRC #39,423 #42 and 421 #38 were all processed at the same time we can assume that any differences between these samples can be correlated to the material. The Io is the same in samples 423 and NRC but five orders of magnitude greater than in sample 421, which has an Io of IO' 1 2 . The difference between 60 these two sets of samples is that in 421 the doping starts 0.03 |im away from the quantum well, while in the others it is at least 0.23 nm away. The difference in Io is possibly due to the fact that in materials 423 and NRC there is a considerable amount of AlGaAs in the depletion region of the diode. AlGaAs is known to incorporate more oxygen and other deep level contaminants than GaAs 5, so it would be reasonable to guess that the lifetime would be shorter in the depletion region of materials 423 and NRC. Furthermore, in the case of the 423 material, the AlGaAs is doped with carbon which is also believed to introduce deep level contaminants. According to equation 4.2 and 4.3 a shorter lifetime would suggest a larger Io. The diode ideality factors obtained are more difficult to explain since the standard theory does not predict diode idealities greater than two. We note that as the recombination in the depletion region increases the diode ideality factor increases from 1 to 2 according to the theory. In an attempt to model diode idealities greater than two we have determined an expression for the current through two diodes in series. The current flowing through two diodes as a function of high forward bias (V) is: / \u00E2\u0080\u00A2 V V e ^ > T (4.4) where ni, n2,and 701 , I02 are the diode ideality factors and reverse saturation currents respectively If either ni or n2 are greater than 1 then two diodes in series will appear to be a single diode with a diode ideality greater than 2. We have applied this idea to our structure to see if it is possible to model the laser as two separate diodes in series. One possibility is that the second diode is a Schottky barrier diode at one of the contacts. This explanation, however, is unsatisfactory because it does not predict the variation from material to material. We would expect the diode ideality factors to be the same for materials with identical contact layers (421 and 423) that were processed at the same time (runs #38 and #42). This is not the case as n for 423 is twice as 61 large as that for 421. Furthermore, for both NRC and 423 the diode ideality seems to be independent of the processing run, even where the Io values differ by four orders of magnitude. We therefore conclude that a Schottky barrier at one of the contacts is probably not responsible for the diode ideality factors. Another possible explanation follows from the idea that the active region itself can be modeled as two separate diodes. The active region of a laser is not just a simple p-n junction; it is complicated by the presence of various heterostructures, the quantum well, and undoped spacer layers. As a slightly more elaborate model we considered the energy band diagram in figure 4.9, which shows recombination taking place at the quantum well and in the neutral regions to either side. Active Region Figure 4.9: Energy band diagram for a quantum well in a p-n junction. Dashed lines indicate quasi-fermi levels. We suggest that under the right circumstances figure 4.9 can be considered electrically to be two diodes in series. One diode an nli junction, where holes are injected from the quantum well into the n-type material and the other is an ilp junction, where electrons are injected 62 from the quantum well. It should then be possible to obtain diode ideality factors greater than two by introducing n>l ideality factors for the diodes. For this analysis to apply the quasi-Fermi level separation should be small in the quantum well, so that the minority carriers can be considered to originate in the quantum well. In this interpretation large diode ideality factors would be expected when the quasi-Fermi level separation is relatively large in the nli and ilp junctions, and relatively small in the quantum well. In the limit of no quasi-Fermi level separation in the quantum well the band diagram in figure 4.10 may be obtained. \u00E2\u0080\u0094 \u00E2\u0080\u0094 \u00E2\u0080\u0094 Quasi-Fermi Levels Figure 4.10: Energy band diagram for a quantum well in a p-n junction with no quasi-Fermi level separation in the quantum well. This would be the case if the quantum well were replaced with a metal layer. We might expect this to be the case for large i-layers, where the space between the n-doped and p-doped layers is large. Also a heavily C-doped p-layer will shorten the lifetime in the vicinity of the pli interface which will also tend to reduce the quasi-Fermi level separation in the active region. We would therefore expect the largest diode ideality factor from laser from 423, since for this material the doping is pulled back 0.23 nm from the quantum well. Furthermore this diode is also heavily carbon doped, leading to an increased recombination is the depleted layer. For the NRC material we would expect a diode ideality smaller than 63 material 423 since, while the doping is also pulled back from the quantum well by 0.23 t^m, the diode is beryllium doped which we expect to give lower recombination at the contact. Finally we would expect diodes from 421 to have smaller ideality factors since the doping is within 0.03 |im of the quantum well. An examination of Table 4.7 shows that this is indeed the case. Diode idealities of around 3.3 were measured for material 423, 2.4 for NRC and 1.6 for 421, showing that this model is consistent with the diode ideality factors observed. 4.3 Conclusions There is a considerable amount of information that can be obtained using electrical characterisation methods. We have determined that about 15% of the current in these devices flows around the ridge in both alumina isolated and AlGaAs isolated samples at high bias. The alumina isolation layers seem to have little or no effect at high bias voltages. The contact resistance of the p-type ohmic contact has been determined from series resistance measurements to be (12\u00C2\u00B15) IO - 5 Qcm 2 , which is approximately what is found in the literature29 for the Cr/Au contact we are using. Finally a two diode model has been proposed to qualitatively explain the diode ideality factors greater than 2 encountered in this work. The results presented thus far however do not include any information about light output. From the data presented in this chapter it is not possible to determine which structures lase. The purpose of the next chapter is to describe the optical characterisation that has been carried out on our lasers. 64 5 Ridge Waveguide Laser Optical Characterisation This chapter describes measurements of the optical output of our devices. Light-current characteristics are used to determine the slope efficiency and, most importantly, the threshold current. Turn-on delays can be used to determine the carrier lifetime at threshold and optical spectra give information about emission wavelengths. Following a discussion of these techniques, data for lasers fabricated from NRC material is presented. Only samples from the NRC #30 run were found to lase so this chapter focuses on them. 5.1 Measurement Techniques 5.1.1 Threshold Current and Temperature Dependence The L-I characteristic of a laser is the variation of light emission intensity with drive current. A typical laser L-I characteristic will have three regions. At low current the light output is minimal and quadratic in current, then, at higher current, it appears as if the laser turns on and light output increases linearly with drive current. Finally the emission will be sublinear at high current levels. Figure 5.4 shows the first two regions clearly. Typically the light output would saturate at higher current values than shown in figure 5.4. The current at which the laser appears to 'turn on' is known as the threshold current. Below threshold spontaneous emission dominates the radiative recombination. Spontaneous emission is relatively low in intensity and has a broad spectral output. Above threshold stimulated emission dominates and the light intensity rises rapidly and the emission narrows to distinct lasing modes. An analysis of recombination rates when stimulated emission dominates gives:35 L o c I - J t h (5.1) where L is the optical output power. Above threshold, because of the gain, the photon density is limited only by the number of injected carriers* which is directly proportional to 65 the current. The final region of the curve is the roll off at high drive currents, which is called power saturation. Power saturation is often attributed to heating of the device, leading to a decrease in carrier lifetime or an increase in leakage current.36 The technique we have used for measuring L-I curves uses a current pulse to drive the laser and a photodetector to measure the light. Figure 5.1 shows the circuit set up used to measure L-I curves. Pulse Generator to ta l Figure 5.1: Circuit diagram for L-I measurements A pulsed current was used since we were concerned that high DC currents would destroy the devices. The current was determined from the voltage drop across the series resistor as measured using an oscilloscope. The light was measured using a silicon photodetector with a built in op-amp and a 10 M Q feedback resistor. The output of the photodetector was measured with a DC voltmeter, so the meter value corresponds to a time averaged power. To determine the actual output power of a diode an independent measurement using an 66 FND-100 silicon photodiode and an oscilloscope was made to calibrate the DC voltmeter curve. Curves that have not been calibrated using this method are also presented in this work with light units as 'arbitrary'. The optical alignment of the diode with the detector is important so the set-up in figure 5.2 was developed. Photo Detector Figure 5.2: Apparatus for aligning laser diodes The diode is approximately aligned with the detector and the set-up is covered with dark blankets to reduce the background light. Above threshold pulses, generally 100 ns wide, are applied to the diode, which is then aligned to the position with the largest light signal using the three-axis translation stage. Before data is taken the output of the detector is viewed on the oscilloscope at maximum drive current to ensure that the detector is not saturated. Data is then taken over as large a current range as possible, generally from about 5 mA to about 150 mA. L-I curves are used to determine the threshold current for a laser. Threshold current and threshold current density are two of the most frequently quoted figures of merit for semiconductor lasers. The threshold current is the current where there are sufficient carriers to cause a gain greater than the internal losses. Since the lasing threshold is related 67 to the requirement to build up a certain carrier density it will vary with the volume of the active region. For this reason a more useful parameter for comparing different lasers is the threshold current density which is the threshold current divided by the active region area. It has been difficult to quantify where the current flows in our lasers so, for the remainder of this chapter, I shall assume that all of the current flows through the ridge, and that the leakage current around the ridge is negligible. The active region area is taken to be the ridge area. The threshold current is temperature sensitive, and the nature of the temperature dependence is especially important for communications lasers. To determine the temperature dependence L-I curves are measured at several different temperatures. Control of the temperature to \u00C2\u00B11\u00C2\u00B0C was achieved by mounting the lasers on thermoelectric coolers, monitoring the temperature with a thermocouple and manually adjusting the current supplied to the T E cooler. A schematic showing the set-up of the T E cooler and the thermocouple was given in figure 2.16. A nitrogen flow was used to prevent condensation on samples cooled to below the dew point. The threshold current is expected to vary with temperature as:3 7 X Ith{T) = l / \u00C2\u00B0 (5.2) where T is the temperature, Io is a constant, and To is a characteristic temperature of the laser. A high value of To means that the threshold current of the diode is relatively insensitive to temperature variations. The temperature dependence of the threshold has been measured for only one laser, #30, and the results are discussed in section 5.2.4. 5.1.2 Turn-On delay When a current pulse, greater than threshold, is applied to a laser it does not start lasing immediately since the carrier concentration must build up to reach threshold. The time required for this build up to occur is known as the turn-on delay (td). A typical turn-on 68 delay is on the order of nanoseconds, so a fast oscilloscope and photodetector are used. In this work an FND-100 photodetector biased at 90V (rise time of less than 1 ns) is used to measure the light pulse. The output of the photodetector and the current pulse are both read using a Tektronix 7104 oscilloscope triggered by the current pulse (figure 5.3). 50O \u00E2\u0080\u0094W\A/ Transmission Line -AMM/ 3 3 0 Oscilloscope Pulse Generator FND-100 Photodetector Figure 5.3: Turn-on delay measurement apparatus The turn-on delay is measured directly from the separation of the light and current pulses. When the initial current through the laser is zero and the drive current is much greater than the threshold current the turn-on delay is given approximately by the expression:38 (5.3) d e \ i where td is the turn-on delay, x e is the carrier lifetime at threshold, Ith is the threshold current and I is the drive current. The turn-on delay is measured as a function of the drive current for a laser with a known threshold to obtain the carrier lifetime. 69 5.2 Laser Characterisation This section consists of an analysis of four lasers, #30, #36, #37, and #38. The L-I characteristics and emission spectra for lasers #36-#38 are similar and L-I data, turn-on delay data and optical spectra are presented for these lasers. Qualitatively, lasers #36-#38 performed as one would expect from the literature. Laser #30, on the other hand, displays some considerable departure from 'typical' behaviour. For laser #30 we present temperature dependent L-I data and optical spectra. Spectra taken for laser #30 show evidence of lasing on transitions between the second energy levels in the quantum well. At sufficiently high currents the transitions from the second energy levels dominate the gain. The material properties and current characteristics for the NRC #30 diodes which lased are included in sections 1.2 and 4.2. The relevant properties of these devices are included in table 5.1. The two most important things to note are that laser #30 is twice as wide as the other lasers and it is mounted on a T E cooler. Property #30 #36, #37, #38 Width of Ridge 6^ im 3|im Length of Ridge 240fim 475|im Bonding Method Wire Bond Wire Bond Mounting T E Cooler Copper Block Block #4 #2 Jo 4 10\"8 Acm- 2 IO\"6 Acm- 2 s^eries 9.1 Q ~ 15 Q Rshunt 500 MQ 490 M Q Table 5.1: Laser properties for lasers #30 and #36, #37 and #38 70 5.2.1 L-I characteristics for lasers #36 and #37 Lasers #36 and #37 are of specific interest because they behaved as might be expected for a typical QW laser. The analysis begins with an examination of the threshold current for laser #37. The threshold current and current density are calculated from the best fit shown in figure 5.4. .Q CO 0.02 0.04 0.06 0.08 0.1 Current (Amps) \u00E2\u0080\u00A2 Indicates where spectra have been taken Figure 5.4: L-I curve for laser #37 with a threshold current of 39 mA. The threshold current for laser #37 is 39 mA, which corresponds to a current density of 2.0 kA/cm 2 . Typical values in the literature range from about 350-1000 A/cm 2 H - 3 9 for InGaAs QW ridge waveguide lasers with about the same dimensions as laser #37. We believe that the high threshold current density is due to poor mode confinement. Our lasers were designed to be weakly index guided but a more careful analysis revealed that the index contrast due to the ridges was too small 4 0, so our structures are almost purely gain guided. The lack of index guiding means that the optical mode is quite large, extending well past the region under the ridge. Because of the size of the mode the overlap of the mode with the gain profile, which is confined to the region under the ridge, is relatively small. A small 71 overlap would mean that much of the mode does not experience gain, leading to a higher threshold current. An L-I curve for laser #36 has also been measured. Figure 5.5 shows the L-I curve with the best fit line to the linear portion. 1.2 1 h ^ 0.8 \u00E2\u0080\u0094 0.6 h \u00E2\u0080\u00A24\u00E2\u0080\u0094' \u00E2\u0080\u0094i 0.4 h 0 i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094|\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094|\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094|\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094|\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094|\u00E2\u0080\u0094r I =44 mA J ? th 9 i f \" 1 / s 0.2 V th &o\u00C2\u00BBoo<\u00C2\u00BBoriOri 1 \u00E2\u0080\u0094 I \u00E2\u0080\u0094 i i i I ' i i \u00E2\u0080\u0094 I \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 l _ 0 0.02 0.04 0.06 0.08 0.1 0.12 Current (A) \u00E2\u0080\u00A2 Indicates where spectra have been taken Figure 5.5: L-I curve for laser #36, showing a threshold current of 44 mA The threshold current is 44 mA, and the threshold current density, Jth, is 2.3 kA/cm 2 . The slope efficiency, which is the number of photons emitted from the diode per electron-hole pair injected, can be calculated from the slope of the linear portion in figure 5.5. Laser #36 has an efficiency of about 38 mW/A, or 4.5% assuming that 40% of the emitted light is collected by the photodetector. 40% is a reasonable assumption because most of the light emitted from the front of the laser will be collected by the detector, while all of the light emitted from the back is lost. Typical ridge waveguide QW lasers reported in the literature have quantum efficiencies of about 42%5, which is about one order of 72 magnitude greater than for our structures. This may be due to poor overlap between the optical mode and the electrically injected electron-hole pairs. 5.2.2 Turn-on Delay and Carrier Lifetime for Laser #38 The turn-on delay has been measured for laser #38. A plot of turn-on delay (td) versus the ratio of drive current (I) to threshold current (1^ ) is included as figure 5.6. 7 6 5 \u00C2\u00A3 . 4 h o 3 h 2 F-i ik i i i i i | i i i i i i i i i | i i i i i i i i i | I I I I ^ M I I I | I I I I \ - T \u00E2\u0080\u0094 r T = 11 ns 11 1 1 1 i \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 11 1 1 1 11 1 1 1 i 1.5 2.5 l/l th Figure 5.6: Turn-on delay (tr transitions). The calculation predicts two allowed transitions in the quantum well. It predicts an allowed transition at 1008 nm, which is the 1\u00E2\u0080\u0094>1, and an allowed 2\u00E2\u0080\u0094>2 transition at a wavelength of 920 nm. The 1-\u00C2\u00BB1 transition at 1008 nm is quite close to the laser peaks in figure 5.13, which are around 1000 nm. The 2\u00E2\u0080\u0094>2 transition threshold wavelength of 920 nm is quite close to the wavelength of 915 nm observed for the second mode in figure 5.14. A first approximation of the gain in the NRC material is calculated next, which shows that the 2->2 transition can experience gain at high currents. The gain profile was calculated as a function of carrier density and wavelength for a quantum well with the parameters from the theory given above. The optical gain is given Where A is a constant, oc(e) is the absorption, Dj(e) is the optical joint density of states, /v(e) and /c(e) are the valence and conduction band fermi occupation functions respectively, and e is the transition energy. The optical joint density of states for a quantum well was given in equation 5.7. We have calculated the gain using these equations, the by:*l a(e) = ADJ(e){fv(e)-fc(e)) (5.8) 84 energies from the Marzin calculation, and the values in table 5.2. The end result is an absorption that is dependent on the number of carriers and the wavelength. We have calculated the gain at typical carrier concentrations for the operation of a ridge waveguide laser. A plot of these gains is included in figure 5.16. 1.5 h Carrier Density 1 10 18 - H \u00E2\u0080\u0094 5 10 18 7 10 1 10 18 19 Absorption Li I I I I I L 900 950 1000 1050 Wavelength 1100 Figure 5.16: Quantum well laser gain profile with different carrier densities In figure 5.16 we can see that at a carrier density of 1 10 1 8 there is gain only around l|im. However, at a carrier density 5 10 1 8 the transition around 920 nm begins to experience gain. The gain around 920 nm increases with the number of carriers and at a carrier density of 7 10 1 8 the gain is slightly higher at 920 nm than it is at the original 1000 nm. At a carrier density of 10 1 9 the gain is greater at 920 nm by a factor of two. This increase in gain is due to carriers populating the second quantum subband in the quantum well. The optical joint density of states is a factor of two higher at an energy above the energy of the second subband. When the occupation is high enough to populate these levels the maximum gain is higher, due to the higher density of states. 85 This can be used to explain the qualitative behaviour of the L-I curves shown in figure 5.10. An examination of the output spectra presented in figures 5.13 and 5.14 shows that the high current regime begins when there is gain at the lower wavelength. To make clear the agreement of the gain calculations with the observed emission spectra the theoretical gain at a carrier density of 7 10 ^ has been plotted over top of the emission spectra at 250 mA. jQ c o CO CO ' E LU Gain \u00E2\u0080\u00A2 Emission -J 1.5 \"o -to to ^ 1 0 a-H 0.5 0 850 900 950 1000 Wavelength 1050 Figure 5.17: Emission spectrum at 250 mA with calculated gain spectrum for a carrier density of 7 10 1 8 cm - 3 . In figure 5.17 it can be seen that the onset of this second laser line is probably due to 2->2 transitions. If this mechanism is to explain the L-I curve shown as figure 5.12 we would expect to see some gain at 920 nm with a current of 170 mA since this is where the L-I intensity begins to increase. Figure 5.18 shows the spectrum at 170 mA along with a gain spectrum for a carrier density of 5 101 8. 86 900 950 1000 Wavelength Figure 5.18: Optical emission spectrum at 170 mA with gain spectrum for n=5 10 1 8 cm - 3 . Inset shows expanded region around 915 nm where there is an increase in the emission spectrum corresponding to 2->2 transitions. Inset in this plot is an expanded view of the region around 915 nm, where there is clearly an increase in the intensity of the emitted light. At lower currents, 80 mA and 20 mA the output at the lower wavelength is at background levels. 5.2.6 Electroluminescence for Laser #30 Electroluminescence has been measured at 20 mA for three different temperatures, -10\u00C2\u00B0, 0\u00C2\u00B0, and 31\u00C2\u00B0C. The theoretical E L emission spectrum was discussed in section 5.2.3, and such theoretical curves have also been calculated for the three temperatures measured here. Figure 5.19 shows the theoretical results with the experimental data for the E L at 31\u00C2\u00B0C. 87 r \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 | \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 | \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 | \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 | \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 r 850 900 950 1000 1050 1100 Wavelength (nm) Figure 5.19: Electroluminescence from laser #30 with theoretical emission spectrum calculated using equation 5.6. The junction temperature is taken to be 31\u00C2\u00B0C, the temperature on the sample thermocouple. The theoretical curve has been scaled to match the data at 1010 nm. Once again we see good agreement between the experimental data and the theoretical curve. The agreement at other temperatures is also good. 5.3 Conclusion This chapter has discussed some general techniques that may be applied to the optical characterisation of semiconductor lasers. We have characterised the optical output of several laser structures from NRC #30. Room temperature threshold current densities of about 2 kA/cm 2 (#36) have been obtained. These values are high for ridge waveguide lasers since typical results in the literature for strained layer InGaAs quantum well ridge waveguide lasers range from about 350 A / c m 2 1 1 to about 1000 A/cm 2 3 9 for lasers 400 to 500 (im long and about 3 [im wide. We feel that the higher threshold values obtained are due to the poor optical confinement present in the lasers due to insufficient index guiding 88 from the ridge. The slope efficiency of laser #36 was determined to be 4.5%, which is again low as compared to typical results of 42% in the literature5. The temperature dependent behaviour of laser #30 was also determined. To was measured to be 88 K below 10\u00C2\u00B0C and 28 K above 10\u00C2\u00B0C. The shape of the L-I curves and the shift in To from 88 to 28 K may be due to carrier injection into the second quantum subband. This type of band filling behaviour has also been reported in the literature for strained layer InGaAs quantum well ridge waveguide lasers, particularly when the ridges are short5. 89 6 Conclusion In this work we have presented the procedure that we have used to fabricate InGaAs quantum well ridge waveguide lasers on substrates grown by M B E at UBC and NRC. We have characterised several aspects of the processing procedure, and have presented a discussion of the ridge formation, and the ohmic contacts. The have also measured some of the electrical and optical characteristics of our devices. The formation of the ridges using electron beam lithography and PN-114 resist has been very successful. Acceptable ridge profiles, good resistance to etching and write times of around one minute have all been achieved. Directly writing the patterns with the electron-beam also allows flexibility in ridge length, width, and density on a sample that is unavailable in optical masks. Electrical, I-V, measurements have been used to characterise the contact resistance of the p-type contact and the quality of the electrical isolation from the peripheral semiconductor materials. Using AlGaAs contacts for isolation we have obtained similar shunt current densities as in the literature24. The geometry of our device however leads to two orders of magnitude more current flowing around the ridges in our devices. An alumina isolation layer on the AlGaAs was found not to increase the electrical isolation. This result has not been adequately explained. Typical series resistances measured are around 10 Q, which is similar to other ridge waveguide lasers in the literature11. Shunt resistances varied greatly from sample to sample and run to run, and typical values ranged from 100 kQ to 500 M Q . The ohmic contacts we have produced have specific contact resistances of 1.9 10 - 4 Qcm 2 and (12\u00C2\u00B15) 10 - 5 Qcm 2 for the n and p-type contacts respectively, which is equivalent to similar contacts in the literature29. A problem with our ohmic contacts is that the series resistance does not dominate until high bias for several of our devices. While we have established that this effect is not due to low values for Io, we are currently unsure of the mechanism for this high resistance. o 90 We have found that it is possible to correlate the doping profile in the region of the quantum well with the diode ideality factor of the device. Diodes with doping close to the quantum well can have diode ideality factors half that of diodes with the doping pulled back from the quantum well. A two diode model is proposed to explain this correlation and the occurrence of diode ideality factors greater than two. Further quantitative analysis is needed to confirm this interpretation. We have shown that our fabrication technique is capable of producing functioning lasers by producing several operating lasers from NRC material. However, since only NRC #30 produced functioning lasers, greater reproducibility is required. Our most successful lasers had ridges about 475 |j,m long and 3 |im wide with threshold current densities of ~2 kA/cm 2 and slope efficiencies of about 4%. Typical laser in the literature about 500 fim long and 3 |im wide have threshold current densities of 1 kA/cm 2 3 9 and slope efficiencies of about 42%5. We have measured the To value for one laser and obtained a value of 88 K below 10\u00C2\u00B0C and 28 K above that temperature. A typical To value from the literature is about 140 K 5 . The change in To has also been correlated to the onset of lasing in the second subband of the laser. We have determined that the optical confinement in our structures is poor, which could explain the high threshold currents and the low slope efficiencies. The performance of our laser is believed to be worse than the literature due to poor optical mode confinement. The optical confinement can be improved by increasing the height of the ridges to increase the effective index of refraction contrast in our structures. Effort must be put into determining the mechanism behind the poor reproducibility in our procedure. SIMS work is currently under way to determine the impact of our processing technique on the active region of the laser structures. Contacting schemes using rapid thermal processing are being examined to remove the long high temperature steps in the process. Also, more work is required to determine why the alumina isolation layers do not improve the isolation in our devices. Finally, work is underway to use the processes 91 outlined in this work to produce distributed feedback lasers on substrates with gratings the active region. 92 References 1. Govind P. Agarwal and Niloy K. Dutta, Long-Wavelength Semiconductor Lasers (2nd Ed), Van Nostrand Reinhold, New York, 1993, pg. 1-5 2. Govind P. Agarwal and Niloy K. Dutta, Long-Wavelength Semiconductor Lasers, Van Nostrand Reinhold, New York, 1986 3. Ibid. pg. 51 4. S. R. Johnson, C. Lavoie, M . K. Nissen, T. Tiedje, US patent 5,388,909 (1995) 5. Naresh Chand, Sung Nee George Chu, Niloy K. Dutta, John Lopata, Michael Geva, Alexei V. Syrbu, Alexandra Z. Mereutza, and Vladimir P. Yakovlev, IEEE Journal of Quantum Electronics 30,424 (1994) 6. M . D. Johnson, C. Orme, A. W. Hunt, D. Graff, J. Sudijomo, L . M . Sander, and B. G. Orr, Phys. Rev. Let. 72, 116 (1994) 7. Christian Lavoie, Ph.D. Thesis (University of British Columbia, 1994) 8. M . Micovic, P. Evaldsson, M . Geva, G. W. Taylor, T. Vang, and R. J. Malik, Appl. Phys. Lett. 64, 411 (1994) 9. A. Busch, M.Sc. Thesis (University of British Columbia, Vancouver, 1994) 10. Ralph Williams, Modern GaAs Processing Methods, Artech House, Boston, 1990, pg. 101-106 11. F. Vermaerke, P. Van Daele, G. Vermeire, I. Moerman, and P. Demeester, SPIE 1851, 23 (1993) 12. R. Morin, M.ASc. Thesis (University of British Columbia, Vancouver, 1995) 13. B.L. Sharma Ohmic contacts to III-V Compound Semiconductors, in Semiconductors and Semimetals Volume 15 Contacts, Junctions, Emitters, Edited by: Robert K. Willardson and Albert C. Beer, Academic Press New York 1981 14. A. Piotrowska and E. Kaminska, Thin Solid Films 193/194, 511 (1990) 15. R. K. Kupka and W. A. Anderson, J. Appl. Phys. 69 (6), 3623 (1991) 93 16. V. L . Rideout, Solid-State Electronics 18, 541 (1975) 17. C. A. Mead and W. G. Spitzer, Phys. Rev. 134, A713 (1964) 18. A. M . Cowley and S. M . Sze, J. Appl. Phys. 36, 3212 (1965) 19. J. M . Woodall, G. D. Pettit, T. N. Jackson, C. Lanza, K. L . Kavanagh, and J. W. Mayer, Phys. Rev. Lett. 51, 1783 (1983) 20. F. A. Padovani and R. Stratton, Solid-State Electronics 9, 695 (1966) 21. Peter A. Barnes, SPIE 1632 Optically Activated Switching JJ, 98 (1992) 22. A. Y. C. Yu, Solid-State Electronics 13, 239 (1970) 23. D. A. Fraser, The Physics of Semiconductor Devices (3rd Ed), Clarendon Press, Oxford, 1983 24. M . C. Amann, Electronics Letters 15 (14), 441 (1979) 25. J. Ding, J. Washburn, T. Sands, and V. G. Keramidas, Appl. Phys. Let. 49 (13), 818 (1986) 26. R. Cao, K. Miyano, I. Landau, and W. E. Spicer, J. Vac. Sci. Technol. A 8 (4), 3460 (1990) 27. K. Kajiyama. Y. Mizishima, and S. Sakata, Appl. Phys. Lett. 23 (8), 458 (1973) 28. Arlene Wakita, Nick Moll, Alice Fischer-Colbrie, and William Stickle, J. Appl. Phys. 68 (6), 2833 (1990) 29. Haruhiro Matino and Makoto Tokunaga, J. Electrochem. Soc. 116, 709 (1969) 30. Y. K. Fang, C. Y. Chang, Y. K. Su, Solid-State Electronics, 22, 933 (1979) 31. S. M . Sze, Physics of Semiconductor Devices (2nd Ed), John Wiley & Sons, New York, 1981, pg. 89-92 32. S. M . Sze, Physics of Semiconductor Devices (2nd Ed), John Wiley & Sons, New York, 1981 33. Ibid. pg. 84-87 34. Manoj Kanskar (private communication) 94 35. Govind P. Agarwal and Niloy K. Dutta, Long-Wavelength Semiconductor Lasers, Van Nostrand Reinhold, New York, 1986, pg. 56 36. Ibid. pg. 53-60 37. Ibid. pg. 128 38. Ibid. pg. 241-243 39. S. D. Offsey, W. J. Schaff, P. J. Tasker, and L. F. Eastman, IEEE Transactions of Electron Devices 36 (11), 2608 (1989) 40. Manoj Kanskar (Private communication) 41. Govind P. Agarwal and Niloy K. Dutta, Long-Wavelength Semiconductor Lasers, Van Nostrand Reinhold, New York, 1986, pg. 131 42. Ibid. pg. 225 43. Bahaa E. A. Saleh and Malvin Carl Teich, Fundamentals of Photonics, John Wiley & Sons, New York, 1991, pg. 581-610 44. M . Beaudoin, A. Bensaasa, R. Leonelli, P. Desjardins, R. A. Masut, L . Isnard, A Chennouf, and G. L'Esperance (submitted to Phys. Rev. B) 45. Mario Beaudoin (Private communication) 46. Claude Weisbuch and Borge Vinter, Quantum Semiconductor Structures -Fundamentals and Applications, Academic Press, Boston, 1991, pg. 65 95 Appendix A: Component Drawings Laser Mounting Block 4-40 c l e a r 0.900 -0.250 2 - 5 6 t a p ,125 ~T1 T i -ll II .063 .250 96 Packaging Circuit Board _ i n e s 100 w i d e 97 0.180 "@en . "Thesis/Dissertation"@en . "1995-11"@en . "10.14288/1.0074506"@en . "eng"@en . "Engineering Physics"@en . "Vancouver : University of British Columbia Library"@en . "University of British Columbia"@en . "For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use."@en . "Graduate"@en . "Fabrication and characterisation of ridge waveguide InGaAs quantum well lasers"@en . "Text"@en . "http://hdl.handle.net/2429/4306"@en .