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GaAs heterostructures
Thin Solid Films 311 Ž1997. 7–14
Investigation of dislocations and traps in MBE grown p-InGaAsrGaAs heterostructures A.Y. Du a , M.F. Li a
a,)
, T.C. Chong a , S.J. Xu a , Z. Zhang b, D.P. Yu
b
Centre for Optoelectronics, Department of Electrical Engineering, National UniÕersity of Singapore, Singapore 119260 Singapore b Beijing Laboratory of Electron Microscopy, Chinese Academy of Sciences, Beijing P.O. Box 2724, Beijing 100080, China Received 29 January 1997; accepted 21 May 1997
Abstract Dislocations and traps in MBE grown p-InGaAsrGaAs lattice-mismatched heterostructures are investigated by Cross-section Transmission Electron Microscopy ŽXTEM., Deep Level Transient Spectroscopy ŽDLTS. and Photo-luminescence ŽPL.. The misfit dislocations and the threading dislocations observed by XTEM in different samples with different In mole fractions and different InGaAs layer thickness generally satisfy the Dodson–Tsao’s plastic flow critical layer thickness curve. The XTEM, DLTS and PL results are consistent with each other. The threading dislocations in bulk layers introduce three hole trap levels H1, H2 and H5 with DLTS activation energies of 0.32 eV, 0.40 eV, 0.88 eV, respectively, and one electron trap E1 with DLTS activation energy of 0.54 eV. The misfit dislocations in relaxed InGaAsrGaAs interface induce a hole trap level H4 with DLTS activation energy between the range of 0.67–0.73 eV. All dislocation induced traps are non-radiative recombination centers which greatly degrade the optical property of the InGaAsrGaAs layers. q 1997 Elsevier Science S.A. Keywords: Dislocations; GaAs heterostructures; Lattice
1. Introduction The lattice mismatched In xGa 1yx AsrGaAs system has a considerable potential for the fabrication of heterojunction devices such as high electron mobility transistors ŽHEMTs. w1x, heterojunction bipolar transistors ŽHBTs. w2x and strain quantum well lasers w3x. However, several problems associated with lattice-mismatched structures still need to be studied. When the lattice mismatch is accommodated only by the coherent elastic strain, the structure is termed pseudomorphic. When the mismatch is accommodated by both the elastic strain and formation of misfit dislocations, it is named relaxed structure. The boundary between these two structures is not clear. The crossover is estimated by the concept of critical thickness w4x. In the relaxed structure, dislocations introduce deep traps which frequently act as recombination centers or scattering sites.
)
Corresponding author.
0040-6090r97r$17.00 q 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 3 1 4 - 3
To avoid degradation of device performance Žsuch as HEMTs. due to dislocations, pseudomorphic structures with thin dislocation-free InGaAs layers are widely used for fabricating devices. However, device designing becomes more difficult as the available layer thickness becomes thinner with the increase of In mole fraction in the case of pseudomorphic structures. Therefore, there is a need to grow layers thicker than critical thickness. On the other hand, despite the presence of interface dislocations, Ramberg et al. w5x demonstrated that the higher-gain HBTs devices could be obtained for a higher indium mole fraction Ž x s 0.08. sample due to the increase in D E v , the valence band offset. Hence, a better understanding of degradation in the electrical or optical properties of relaxed In xGa 1yx As is of great importance for future growth and optimization of practical devices in the In xGa 1yx AsrGaAs system. Recently, a number of experiments were carried out to investigate the electrical traps associated with misfit dislocations in the InGaAsrGaAs system w6–10x, but most of the studies were emphasized on the electron traps in n-InGaAsrGaAs.
A.Y. Du et al.r Thin Solid Films 311 (1997) 7–14
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The samples were grown using a RIBER MBE 32P molecular-beam epitaxy ŽMBE. system. The first 0.2 m m nq GaAs buffer layer of each sample was grown at a substrate temperature of 6008C, the other layers were grown at 5508C. The temperature was measured by a pyrometer. The growth rate was about 1 m mrh, and the reflection high-energy electron diffraction ŽRHEED. method was used to monitor the growth conditions. All layers were grown on nqy GaAs Ž001. substrates. The nqy GaAs buffer layer was doped with Si Ž) 1 = 10 18 cmy3 .. All p-type GaAs and InGaAs layers were doped with Be Žf 1 = 10 17 cmy3 .. The composition of InGaAs layer is measured by Philips DCD-3 double crystal X-ray diffractometer after growth. The cross-section transmission electron microscopy ŽXTEM. was used to observe the distribution of dislocations, and the electrical traps were measured by deep-level transient spectroscopy ŽDLTS.. The optical properties of pseudomorphic and relaxed InGaAs layers were characterized by photoluminescence ŽPL. technique. The XTEM’s specimens were prepared by standard processing technique, and a final thinning was carried out by ion milling on a cooled stage using low Ar ion intensities. The samples were observed using a Philips EM 420 operating at 100 KeV, and the high resolution electron microscopy ŽHREM. was performed using a JEOL 2010 operating at 200 KeV. For DLTS measurement, the diode devices were fabricated by conventional lift-off and wet chemical etching processes. Ohmic contacts were formed by evaporating AurNirAuGerNi on the nqy GaAs substrates and AurCr on the pqy GaAs Žor InGaAs. layers, and annealing was carried out at 4508C for 25 s. The BIO-RAD DL 4600 DLTS system was used to measure the diodes with circular p–n junction area of 400 m m in diameter. Low temperature PL spectroscopy experiments were carried out with the samples mounted in the APD HC-4 ˚ Arq ion laser or 7800A˚ cryostat and excited by 4880A Ti-sapphire laser.
Fig. 1. The In mole fraction = and layer thickness of In x Ga 1yx As layer in four samples; The curve is the Dodson–Tsao critical thickness curve at 5508C growth temperature w11x.
In this paper, we report some new hole traps associated with dislocations in p-InGaAsrGaAs system. Combining the Cross-section Transmission Electron Microscopy ŽXTEM. and Deep-level Transient Spectroscopy ŽDLTS. results, we identify that three hole traps and one electron trap H1, H2, H5 and E1 are associated with threading dislocations, and one hole trap H4 is associated with misfit dislocations. We also observed that the dislocations cause degradation in the optical properties of InGaAs and GaAs materials. 2. Experiments To investigate the electrical and optical properties of dislocations, we have designed carefully the composition and thickness of the InGaAs layers. Fig. 1 shows the In mole fraction x and thickness of In xGa 1yx As layers in the four samples grown at a substrate temperature of 5508C and the curve for the In xGa 1yx As layer critical thickness, according to the Dodson–Tsao plastic flow model w12x. The detailed sample structures are show in Table 1. The cap p-GaAs layer in sample 1, 2, 3 is used to stop Be out-diffusion in pq layer.
Table 1 The structures of four samples Sample 1
2
3
4
˚. p GaAs Ž200 A Ž . p-GaAs 1 m m
˚. p GaAs Ž200 A ˚ Ž p-GaAs 2000 A.
˚. p GaAs Ž200 A Ž . p-GaAs 1 m m
˚. p-In 0.05 Ga 0.95 As Ž500 A p-GaAs Ž0.1 m m.
˚. p-In 0.12 Ga 0.88 As Ž500 A p-GaAs Ž0.1 m m.
˚. p-In 0.24Ga 0.76 As Ž650 A p-GaAs Ž0.1 m m.
˚. pq In 0.05 Ga 0.95 As Ž200 A Ž . p-In 0.05 Ga 0.95 As 2 m m p-GaAs Ž0.1 m m.
˚. GaAs Žundoped 200 A nq GaAs Žbuffer 0.5 m m. nq Sub GaAs
˚. GaAs Žundoped 200 A nq GaAs Žbuffer 0.5 m m. nq Sub GaAs
˚. GaAs Žundoped 200 A nq GaAs Žbuffer 0.5 m m. nq Sub GaAs
˚. GaAs Žundoped 200 A nq GaAs Žbuffer 0.5 m m. nq Sub GaAs
q
q
q
The Ž001. nq GaAs substrates are Si doped with doping concentration 1 = 10 18 cmy3 . The nq GaAs buffer layers are Si doped with doping concentration 2 = 10 18 cmy3 . The p-GaAs or p-InGaAs layers are Be doped with doping concentration 1 = 10 17 cmy3 . The pqy GaAs or InGaAs cap layers are Be doped with doping concentration above 2 = 10 18 cmy3 .
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3. Results and discussion 3.1. A. InÕestigation of structure defects and dislocation distribution by TEM Four samples were studied by Cross-sectional Transmission Electron Microscopy ŽXTEM. to investigate the structure defects associated with lattice mismatch. The main structural defects are misfit dislocations and threading dislocations. Most misfit dislocations in the InGaAsrGaAs system are 608 dislocations with four 1r2²110: Burgers vector inclined at 458 to the Ž001. interface plane w13x. In the InGaAsrGaAs system Žepilayer under compression., any 608 dislocation at interface has the extra half plane lying in the GaAs substrate. In this paper, we define the misfit dislocation as the 608 dislocations at the interface. Other dislocations are defined as threading dislocations. 3.2. Sample 1 We performed XTEM on four specimens of sample 1 and found no dislocations in them. Fig. 2a is an XTEM picture of sample 1 and the sample structure is shown in Table 1, sample 1. Since the InGaAs layer in our sample 1 is much thinner than the critical thickness Žsee Fig. 1., misfit dislocations should not be found in the sample. No defects Ždislocation, stacking faults. were observed in the four specimens by TEM even when the micrograph areas were moved around several m m in each specimen. In Fig. 2a, the strain of the interfaces were studied. There are two sharp interfaces of InGaAs layer, but the contrast of the lower InGaAsrGaAs interface is much sharper than the upper interface, and the bend contour in GaAs layer is shifted more at the lower interface than at the upper interface. The above results shows that there are stronger strain at the lower interface than at the upper interface. On the other hand, the strain at the interfaces causes the dark contrast band near the interface. At the lower interface, the dark contrast band as well as the strain are distributed symmetrically on both sides of the interface. However, the dark contrast band and the strain at the upper interface are almost distributed only in GaAs cap layer. 3.3. Sample 2 Three types of dislocations were found in sample 2 and their distributions were also studied. The structure of sample 2 is shown in Table 1, sample 2 and the corresponding XTEM picture of this sample is shown in Fig. 2b. Owing to the GaAsrInGaAsrGaAs sandwich structure of sample 2, there are two misfit dislocation types in this system: the dislocation dipole and the single-dislocation w13,14x. The dislocation dipole is a pair of parallel 608 dislocations with Burgers vectors of opposite sign, lying in
Fig. 2. XTEM micrographs of the four samples shown in Fig. 1. Ža. The bright-field image of sample 1 shows no dislocation in the sample. Žb. The dark-field image of sample 2 shows the dislocations and their distribution in the sample. Žc. The weak-beam image of sample 3 shows the very high density of threading dislocation in InGaAs layer and dislocations in cap GaAs layer in the sample. Žd. The dark-field image of sample 4 shows the dislocations and their distribution in the interface and lower GaAs layer in the sample. The stripes in InGaAs and GaAs layers are the thickness fringes of the specimen.
the same 1114 glide plane, one at each interface. The single-dislocation which is not part of a dipole only appears at lower interface. Fig. 2b shows the two types of misfit dislocations, one is the dislocation dipole, marked as D, and the other is the single-dislocation, marked as S. In Fig. 2b, we also note that there is no dislocation in the cap GaAs layer, however there are some threading dislocations in the lower GaAs layer. According to Lefebvre et al. w15x, segments of reacted misfit dislocations inclining towards the substrate are due to the fact that misfit stress forces are generally higher than the interaction
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elastic force in InGaAsrGaAs system. These segments of misfit dislocations in GaAs layer are defined as threading dislocations in this paper. The threading dislocations only appear in the lower GaAs layer in Fig. 2b and can be explained as follows. All the dislocations at the upper interface belong to dislocation dipoles, however some dislocations at the lower interface are single dislocations. The interaction forces between the pair of dislocation dipoles stop the segments of dislocation inclining towards the upper or lower GaAs layers. Only segments of reacted single dislocations can incline towards the GaAs substrate to form the threading dislocations. 3.4. Sample 3 The structure of sample 3 is shown in Table 1, sample 3. In this sample, the In mole fraction x of In xGa 1yx As layer is about 0.24 and there is a large lattice difference Ž1.7%. between the GaAs layer and the InGaAs layer. With increasing lattice mismatch, the MBE growth mode changes from a two-dimension Ž2D. mode to a three-dimension Ž3D. mode. It was reported by Tabuchi et al. w16x that the 2D growth mode predominates in the In xGa 1yx As layer with In mole fraction x less than 0.5. Ceschin and Massies w17x has shown that the growth undergoes a transition from 2D to 3D if the In composition x is greater than 0.2. The TEM picture of our sample 3 shows that the 2D growth mode dominates in the InGaAs layer, although the In mole fraction x is larger than 0.2. Therefore, our work supports Tabuchi et al.’s conclusion and is in conflict with the work of Ceschin and Massies. The XTEM picture of the sample is shown in Fig. 2c. Fig. 2c is the weak beam dark field image. It shows that in the InGaAs layer there are no stacking faults or micro-twins which are the typical defects in 3D growth epitaxy layer. Therefore, the InGaAs layer is grown with 2D growth mode. Fig. 2c also shows that there is a very high density of threading dislocations in the InGaAs layer. Correspondingly, the quality of the cap GaAs layer, which has many defects, is poor. The stacking faults and threading dislocations are also found in the cap GaAs layer in the XTEM picture. However, the threading dislocation density in the cap GaAs layer is much lower than that in the InGaAs layer. The density of threading dislocations in the InGaAs layer is so high that the reacting dislocations cannot go into the lower GaAs substrate as in sample 2. The lower GaAs layer is dislocation free for sample 3. Fig. 3 is a high-resolution image in the ²110: projection of the InGaAsrGaAs interface of sample 3. Three phenomena were found as below. Ž1. Dislocations are observed at interface in sample 3. These dislocations are threading dislocations shown in Fig. 2c, not the 608 misfit dislocations with regular cross-grid
Fig. 3. High-resolution TEM image of sample 3 shows two dislocations with the extra half plane lying in GaAs layer.
configurations that appeared in sample 2. However, the projected Burgers vector b of the dislocations in sample 3 is the same as the 608 misfit dislocation’s Burgers vector b. Ž2. Islands which are usually observed at 3D growth mode interface are not found in the image and in the InGaAs layer which indicate that there are no stacking faults and micro-twins which are the typical defects in 3D growth mode layer. Fig. 3 shows again that a 2D layer-bylayer growth mode dominates in InGaAs layer. Ž3. Two dislocations with the same Burgers vector b are found and they are so close to each other that the distance ˚ This indicates that the density between them is only 50 A. of threading dislocations in the InGaAs layer is very high. 3.5. Sample 4 The structure of sample 4 is shown in Table 1, sample 4 and the XTEM picture is shown in Fig. 2d. In this sample, no dislocations can be observed in the InGaAs layer except at the interface. The misfit dislocations are located in the interface, similar to the single dislocations in sample 2. The segments of interaction dislocations incline towards the GaAs substrate to form threading dislocations. In this sample, there are some threading dislocations in the GaAs buffer layer which are indicated by arrows in the picture. These threading dislocations propagate into GaAs layer as deep as 0.9 m m below the InGaAsrGaAs interface. In summary, the XTEM pictures of the above four samples show the following.k Sample 1 is dislocation free, but there are strains in the both InGaAsrGaAs interfaces. In samples 2 and 4, there are 608 misfit dislocations. There are some single dislocations along the lower InGaAsrGaAs interface and there are some threading dislocations inclining toward the GaAs substrate. The InGaAs layer in sample 4 is dislocation free. For the sandwich structure in sample 2, there are also dislocation dipole pairs along the upper and lower InGaAsrGaAs interface.
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The dislocation dipoles do not induce any threading dislocations inclining toward the GaAs layer. Therefore, the cap GaAs layer in sample 2 is dislocation free. For sample 3, although the lattice mismatch between In 0.24 Ga 0.76 As and GaAs is as large as 1.7%, the growth mode in InGaAs layer is still two-dimensional. There is a high density of threading dislocations in InGaAs layer. Correspondingly, the quality of the cap GaAs layer is poor, with many defects Žstacking faults and threading dislocations. in the cap layer. No misfit dislocation is found in sample 3. The GaAs substrate is free of dislocations. 3.6. InÕestigation of traps by DLTS measurements Four kinds of diode are used in DLTS measurements. The diodes are made from the same sample structures which have been studied by XTEM, as indicated in Table 1. All diodes used in DLTS measurement have the same area Ž0.1257 mm2 . of p–n junction. As is well known in DLTS measurement, by using different reverse bias voltage Vr and filling pulse height V h , we can detect the trap signal contributed from traps located in different depth regions w18x. When the edge region effect of the deep trap level is considered w19x, the actual active DLTS region is slightly modified and extended deeper to the depletion region. By using different bias and filling pulse voltage, we have different active DLTS region and therefore we can distinguish different traps located in different layers. By this method, we can correlate the relationship between different DLTS trap signals and the corresponding structure defects found in XTEM studies. Fig. 4 shows some typical DLTS spectra of samples 1–4. Six hole traps H1, H2, H3, H4, H5, H1X and two electron traps E1, E2 are found in these samples. Fig. 5 shows the corresponding Arrhenius plots of hole traps DLTS measurements. The hole trap activation energies are calculated from the Arrhenius plot and are listed in Table 2. H1 and H2 hole traps: We identify H1 and H2 as hole traps associated with threading dislocations. This is based on the following facts: Ž1. H1 and H2 hole traps have very large DLTS signal in sample 3, comparatively weak signal in sample 4, very weak signal in 2 and no signal in sample 1. Ž2. For sample 3, when the DLTS active region includes the InGaAs region, the H1 and H2 DLTS signal is very large. When the active DLTS region moves up to the GaAs cap region only, the DLTS signal gradually decreases. Ž3. For sample 4, when the active DLTS region includes the lower GaAs region, we can detect the H1 and H2 DLTS signal. When the active DLTS region moves up and only include the InGaAs region, the H1, H2 DLTS signal disappears. Ž4. For sample 2, when the active DLTS region includes the lower GaAs layer, the H2 DLTS peak is very small with a very weak tail at the low temperature side, probably contributed from H1 traps. When the active
Fig. 4. DLTS spectra of four samples with a rate window 200 sy1 ; the solid curves are measured at small reverse bias with large forward bias injection condition. The dotted curves are the typical DLTS spectra without forward bias injection condition. Ža. Sample 1, there is no any DLTS signal when forward bias injection is not applied. Žb. Sample 2, When the active DLTS region move up to the cap GaAs layer, no any DLTS signal is detected. Žc. Sample 3, when the active region of DLTS move up to the cap GaAs layer, the H5 DLTS signal becomes larger. Žd. Sample 4, when the DLTS active region move up to the InGaAs layer, no any DLTS signal is detected. The inset shows DLTS peak height as function of filling pulse duration for hole traps H2 and H4.
DLTS region moves up to exclude the lower GaAs layer, the H2 and H1 trap signal completely disappears. These DLTS results are completely consistent with the threading dislocation distribution detected by XTEM. Hole traps similar to H1 and H2 have been reported by Ashizawa et.al. w6x Ž0.31 eV. and Choi w7x Ž0.16–0.40 eV. that they are associated with dislocations in the lattice-mismatched interface. H4 hole trap: We attribute H4 hole trap to single and dipole misfit dislocations along the InGaAsrGaAs interface. This is based on the following facts. Ž1. H4 DLTS peak only appears in 2 and 4 samples and disappears in 1 and 3 samples. Ž2. For sample 2, H4 DLTS signal only appears when the active DLTS region includes the InGaAs layer and the lower GaAs p-GaAs layer. When the active DLTS region moves up to only includes the cap GaAs region, the H4 DLTS peak disappears. Ž3.
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Fig. 5. Arrhenius plots for observed electron and hole traps in four p-In x Ga 1yx AsrGaAs heterostructure samples.
For sample 4, H4 DLTS signal only appears when the active DLTS region includes the InGaAsrGaAs interface. These DLTS results are completely consistent with the misfit dislocation Žincluding the dislocation dipole and single dislocation. distribution detected by XTEM. H5 hole trap: We identify H5 as another trap associated with threading dislocation in GaAs. This is based on the following facts: Ž1. H5 appears in sample 3. When the active DLTS region moves up to the cap GaAs layer, H5 DLTS signal increases. H5 also appears in sample 4. However, when the active DLTS region moves up and excludes the lower GaAs layer, the H5 signal disappears. Ž2. There is no H5 DLTS signal for sample 1. The DLTS results are consistent with the XTEM picture of threading dislocations. H3 hole trap: H3 is only found in sample 4 when the active DLTS region is in the InGaAs layer. According to sample 4 XTEM picture, the InGaAs layer is free of dislocations. Therefore, H3 hole trap is not likely to be induced by dislocation and its origin is not clear. H1X hole trap: Only found in sample 1 under a very large forward bias injection. Since sample 1 is free of dislocations, H1X is not related to any dislocations. H1X is also not related to InGaAsrGaAs interface, otherwise, it should be detected in other samples too. The origin of H1X hole trap is not clear yet.
E1 electron trap: E1 DLTS signal was only detected in sample 3 under a very large forward bias injection. We tentatively assign this E1 electron trap to be threading dislocation trap, since only sample 3 has high density of threading dislocations Žsee Fig. 2c.. Electron trap similar to E1 has been assigned as a dislocation trap by many authors w8–10x. E2 electron trap: E2 DLTS signal was detected in all samples Ž1, 2, 3, 4., and only appears under the condition of a very large forward bias injection. We note that E2 also appears in sample 1 which is dislocation free, therefore E2 is definitely not induced by dislocations. Another interesting fact is that the DLTS peak temperature of E2 trap is almost unchanged with rate window and the corresponding activation energy of E2 is larger than the energy gap of GaAs. Therefore, E2 energy level is located in the valance band as a resonant level. We tentatively assign the E2 electron trap to be an interface state of GaAsrInGaAs heterojunction. Further study is needed to clarify this E2 electron trap. Finally, we have checked the DLTS peak amplitudes with the DLTS filling pulse duration time t p . The experimental results for the well resolved H2 and H4 DLTS peaks are plotted in the inset of Fig. 4. As indicated in the inset, H2 and H4 peak amplitudes depend on the logarithmic t p . These results are in agreement with previous reports for the dislocation associated traps in GaAs w20,21x and in Si w22x, further supporting our identification in this work. On the contrary, the well resolved DLTS peak amplitude of H1Y remains unchanged within the experimental accuracy when t p changed from 10y4 s to 10y1 s. This also supports that the H1Y trap is not related to any dislocations. In summary, using different bias voltage and pulse height in DLTS measurement, we can detect and distinguish the trap states located in different layers in the samples. Comparing these DLTS information with XTEM pictures, we assign: Three hole trap states H1, H2, H5 associated with threading dislocations. One hole trap state H4 associates with misfit dislocations alone the InGaAsrGaAs interface. One electron trap E1 associates with threading dislocations. One electron trap E2 associates with InGaAsrGaAs interface state. Two hole traps H3 and H1X which are not
Table 2 Hole and electron traps characteristics Trap type X
H1 H1 H2 H4 H5 E1 E2
DLTS activation energy ŽeV.
Identification
0.20 0.32 0.38–0.40 0.67–0.73 0.88 0.54
threading dislocation threading dislocation misfit dislocation threading dislocation threading dislocation
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related to dislocations and the origin is not clear. All the hole and electron trap energy levels are listed in Table 2. 3.7. InÕestigation of dislocation effects by photoluminescence (PL) The low temperature photoluminescence technique is also used to study the effects of dislocation in the four samples. All PL measurements were carried out at sample temperature of 4 K. Fig. 6 shows the PL spectra of samples 1–4, obtained under excitation of Arq laser with power I s 50 mW. We assigned the emission peaks A, B around 1.512 eV and 1.495 eV to band-edge and band-Be acceptor transitions in p-GaAs layer, respectively. The peak D is associated with near band-band transitions in InGaAs layer in different samples. The PL peak positions of InGaAs layers are used to determine the compositions of InGaAs layers in different samples which agree with the results obtained from the Double-crystal X-ray diffraction meter. By comparing the PL spectra of the samples 1–3, it can be seen that the PL intensity is closely related to the dislocation density. The higher the dislocation density in GaAs or InGaAs layer, the lower the PL intensity of GaAs or InGaAs layer. If all Fig. 7. PL spectra at 4 K for sample 1 Ža. and sample 2 Žb. under ˚ Ti–sapphire Laser with different power. excitation of 7800 A
˚ Fig. 6. PL spectra at 4 K for four samples under excitation of 4880 A Arq laser with power I s 50 mW.
the dislocation induced traps are assumed to be non-radiative recombination centers, the PL results are consistent with XTEM and DLTS results. ˚ Further PL explanation should consider that the 500 A InGaAs layer has quantum confinement effects in the 1–3 samples. First, there are high densities of surface states at the GaAs top surface exposed to the air. The PL spectra may be influenced by these surface states. However, due to the quantum confinement effect, the excited electron hole pairs diffuse to and are confined in the quantum well. Therefore, the effect of GaAs surface states are reduced and can be neglected. Second, the PL peak D1 of the InGaAs layer in sample 1, due to the quantum confinement effect, is much higher and narrower than D4 peak of the InGaAs layer in the sample 4 which has 2 m m thickness and almost dislocation-free InGaAs layer. Fig. 7 shows the respective PL spectra of sample 1 and 2, under different excitation power. In Fig. 7a, there are five peaks in sample 1 PL spectra. Peak A1 and B1 are band-edge and band-Be acceptor transitions in p-GaAs layer, peak D1 and E1 are associated with band-edge and band-Be acceptor transitions in InGaAs quantum well. Peak C1 position is strongly dependent of excitation light intensity. Further investigation of peak C1 by photo-luminescence excitation ŽPLE. w23,24x shows that peak C1 is due to the electron transition from the conduction band in the InGaAs to valance band in GaAs near the heterojunction interface. This will be
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reported in another paper in detail. In PL measurement, the quality of the layer is usually characterized by the full-width of half maximum ŽFWHM. of the peak. The FWHM of peaks A1, B1 and D1 are 6 meV at excitation power of 1.0 mW, indicating that the qualities of the InGaAs and GaAs epitaxial layer in sample 1 are good. Fig. 7b shows the PL spectra of sample 2 at different laser power excitations. There are only three peaks A2, B2 and D2 and the positions of all peaks do not change with laser power. Peak D2 ŽInGaAs layer. is so broad that the peak E Žin Fig. 7a. cannot be resolved in the spectra of the sample 2. The energy gap difference between InGaAs and GaAs in sample 2 is larger than that in sample 1, however the InGaAs layer thicknesses in both samples are the same. Therefore, the quantum confinement effect in sample 2 should be stronger than that in sample 1. Although the PL and PLE results show a quantum confinement effect in sample 2, its PL peak intensity of the InGaAs layer is weaker than that in sample 1. The existence of misfit dislocations in InGaAsrGaAs interfaces is the main reason for the degradation of the optical quality of the InGaAs layer in sample 2. The FWHM of the cap GaAs layer A and B are 7.0 meV and 7.0 meV, respectively at excitation power of 1.0 mW and the FWHM of D peak in InGaAs layer is 15.3 meV in sample 2.
4. Conclusions MBE grown p-In xGa 1yx AsrGaAs lattice-mismatched heterostructures with different In mole fraction x and different layer thicknesses are investigated systematically. The misfit dislocations and the threading dislocations observed by XTEM in different samples generally satisfy the Dodson–Tsao’s plastic flow critical layer thickness curve. The XTEM, DLTS and PL results are consistent with each other. By comparing these results, we identify that the threading dislocations in bulk layers introduce three hole traps H1, H2 and H5 with DLTS activation energies of 0.32 eV, 0.40 eV and 0.88 eV, respectively, and one electron trap E1 with DLTS activation energy of 0.54 eV. The misfit dislocations in relaxed InGaAsrGaAs interface induce a hole trap H4 with DLTS activation energy between 0.67–0.73 eV. All dislocation induced traps are
non-radiative recombination centers which greatly degrade the optical property of the InGaAsrGaAs layers. Acknowledgements This work was supported by Singapore National Science and Technology Board RIC-university research project No. 681305. A.Y. Du thanks Prof. K.H. Kuo for the arrangement of TEM work at BLEM. References w1x J.J. Rosenberg, M. Benlamri, P.D. Kirchner, J.M. Woodall, G.D. Pettit, IEEE Electron Dev. Lett. EDL 6 Ž1985. 491. w2x H. Ito, J.S.H. Jun, Electron. Lett. 28 Ž1992. 655. w3x R.J. Fu, C.S. Hong, E.Y. Chan, D.J. Booher, L. Figueroa, IEEE photo. Technol. Lett. 3 Ž1991. 308. w4x R. People, J.C. Bean, Appl. Lett. 47 Ž1985. 322. w5x L.P. Ramberg, P.M. Enquist, Y.K. Chen, F.E. Najjar, L.F. Eastman, E.A. Fitzgerald, K. Kavanagh, J. Appl. Phys. 61 Ž1987. 1234. w6x Y. Ashizawa, S. Akbar, W.J. Schaff, L.F. Eastman, E.A. Fitzgerald, D.G. Ast, J. Appl. Phys. 64 Ž1988. 4065. w7x Y.W. Choi, K. Xie, H.M. Kim, C.R. Wie, J. Electron. Mater. 20 Ž1991. 545. w8x A.C. Irvine, L.K. Howard, D.W. Palmer, Mat. Sci. Forum 83 Ž8. Ž1992. 1291. w9x W.R. Buchwald, J.H. Zhao, M. Harmatz, E.H. Poindexter, Solid-St. Electron. 36 Ž1993. 1077. w10x Y. Uchida, H. Kakibayashi, S. Goto, J. Appl. Phys. 74 Ž1993. 6720. w11x C. Herbeaux, J. Di Persio, A. Lefebvre, Appl. Phys. Lett. 54 Ž1989. 1004. w12x B.W. Dodson, J.Y. Tsao, Appl. Phys. Lett. 51 Ž1987. 1325. w13x T.J. Gosling, R. Bullough, S.C. Jain, J.R. Willis, J. Appl. Phys. 73 Ž1993. 8267. w14x J. Zou, D.J.H. Cockayne, B.F. Usher, J. Appl. Phys. 73 Ž1993. 619. w15x A. Lefebvre, C. Herbeaux, J. Di Persio, Phil. Mag. 63 Ž1991. 471. w16x M. Tabuchi, S. Noda, A. Sasaki, J. Cryst. Growth 99 Ž1990. 315. w17x A.M. Ceschin, J. Massies, J. Cryst. Growth 114 Ž1991. 683. w18x M.F. Li, in: Modern Semiconductor Quantum Physics, ŽWorld Scientific, Singapore, 1994. p. 360. w19x M.F. Li, C.T. Sah, IEEE Trans. Electron Devices ED-29 Ž1982. 306. w20x T. Wosinski, J. Appl. Phys. 65 Ž1989. 1566. w21x G.P. Watson, D.G. Ast, T.J. Anderson, B. Pathangey, Y. Hayakawa, J. Appl. Phys. 71 Ž1992. 3399. w22x P. Omling, E.R. Weber, L. Montelius, H. Alexander, J. Michel, Phys. Rev. B32 Ž1985. 6571. w23x E. Cohen, M.D. Sturge, Phys. Rev. B15 Ž1977. 1039. w24x M.F. Li, In: Modern Semiconductor Quantum Physics, ŽWorld Scientific, Singapore, 1994. p. 189.
Investigation of dislocations and traps in MBE grown p-InGaAsrGaAs heterostructures A.Y. Du a , M.F. Li a
a,)
, T.C. Chong a , S.J. Xu a , Z. Zhang b, D.P. Yu
b
Centre for Optoelectronics, Department of Electrical Engineering, National UniÕersity of Singapore, Singapore 119260 Singapore b Beijing Laboratory of Electron Microscopy, Chinese Academy of Sciences, Beijing P.O. Box 2724, Beijing 100080, China Received 29 January 1997; accepted 21 May 1997
Abstract Dislocations and traps in MBE grown p-InGaAsrGaAs lattice-mismatched heterostructures are investigated by Cross-section Transmission Electron Microscopy ŽXTEM., Deep Level Transient Spectroscopy ŽDLTS. and Photo-luminescence ŽPL.. The misfit dislocations and the threading dislocations observed by XTEM in different samples with different In mole fractions and different InGaAs layer thickness generally satisfy the Dodson–Tsao’s plastic flow critical layer thickness curve. The XTEM, DLTS and PL results are consistent with each other. The threading dislocations in bulk layers introduce three hole trap levels H1, H2 and H5 with DLTS activation energies of 0.32 eV, 0.40 eV, 0.88 eV, respectively, and one electron trap E1 with DLTS activation energy of 0.54 eV. The misfit dislocations in relaxed InGaAsrGaAs interface induce a hole trap level H4 with DLTS activation energy between the range of 0.67–0.73 eV. All dislocation induced traps are non-radiative recombination centers which greatly degrade the optical property of the InGaAsrGaAs layers. q 1997 Elsevier Science S.A. Keywords: Dislocations; GaAs heterostructures; Lattice
1. Introduction The lattice mismatched In xGa 1yx AsrGaAs system has a considerable potential for the fabrication of heterojunction devices such as high electron mobility transistors ŽHEMTs. w1x, heterojunction bipolar transistors ŽHBTs. w2x and strain quantum well lasers w3x. However, several problems associated with lattice-mismatched structures still need to be studied. When the lattice mismatch is accommodated only by the coherent elastic strain, the structure is termed pseudomorphic. When the mismatch is accommodated by both the elastic strain and formation of misfit dislocations, it is named relaxed structure. The boundary between these two structures is not clear. The crossover is estimated by the concept of critical thickness w4x. In the relaxed structure, dislocations introduce deep traps which frequently act as recombination centers or scattering sites.
)
Corresponding author.
0040-6090r97r$17.00 q 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 3 1 4 - 3
To avoid degradation of device performance Žsuch as HEMTs. due to dislocations, pseudomorphic structures with thin dislocation-free InGaAs layers are widely used for fabricating devices. However, device designing becomes more difficult as the available layer thickness becomes thinner with the increase of In mole fraction in the case of pseudomorphic structures. Therefore, there is a need to grow layers thicker than critical thickness. On the other hand, despite the presence of interface dislocations, Ramberg et al. w5x demonstrated that the higher-gain HBTs devices could be obtained for a higher indium mole fraction Ž x s 0.08. sample due to the increase in D E v , the valence band offset. Hence, a better understanding of degradation in the electrical or optical properties of relaxed In xGa 1yx As is of great importance for future growth and optimization of practical devices in the In xGa 1yx AsrGaAs system. Recently, a number of experiments were carried out to investigate the electrical traps associated with misfit dislocations in the InGaAsrGaAs system w6–10x, but most of the studies were emphasized on the electron traps in n-InGaAsrGaAs.
A.Y. Du et al.r Thin Solid Films 311 (1997) 7–14
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The samples were grown using a RIBER MBE 32P molecular-beam epitaxy ŽMBE. system. The first 0.2 m m nq GaAs buffer layer of each sample was grown at a substrate temperature of 6008C, the other layers were grown at 5508C. The temperature was measured by a pyrometer. The growth rate was about 1 m mrh, and the reflection high-energy electron diffraction ŽRHEED. method was used to monitor the growth conditions. All layers were grown on nqy GaAs Ž001. substrates. The nqy GaAs buffer layer was doped with Si Ž) 1 = 10 18 cmy3 .. All p-type GaAs and InGaAs layers were doped with Be Žf 1 = 10 17 cmy3 .. The composition of InGaAs layer is measured by Philips DCD-3 double crystal X-ray diffractometer after growth. The cross-section transmission electron microscopy ŽXTEM. was used to observe the distribution of dislocations, and the electrical traps were measured by deep-level transient spectroscopy ŽDLTS.. The optical properties of pseudomorphic and relaxed InGaAs layers were characterized by photoluminescence ŽPL. technique. The XTEM’s specimens were prepared by standard processing technique, and a final thinning was carried out by ion milling on a cooled stage using low Ar ion intensities. The samples were observed using a Philips EM 420 operating at 100 KeV, and the high resolution electron microscopy ŽHREM. was performed using a JEOL 2010 operating at 200 KeV. For DLTS measurement, the diode devices were fabricated by conventional lift-off and wet chemical etching processes. Ohmic contacts were formed by evaporating AurNirAuGerNi on the nqy GaAs substrates and AurCr on the pqy GaAs Žor InGaAs. layers, and annealing was carried out at 4508C for 25 s. The BIO-RAD DL 4600 DLTS system was used to measure the diodes with circular p–n junction area of 400 m m in diameter. Low temperature PL spectroscopy experiments were carried out with the samples mounted in the APD HC-4 ˚ Arq ion laser or 7800A˚ cryostat and excited by 4880A Ti-sapphire laser.
Fig. 1. The In mole fraction = and layer thickness of In x Ga 1yx As layer in four samples; The curve is the Dodson–Tsao critical thickness curve at 5508C growth temperature w11x.
In this paper, we report some new hole traps associated with dislocations in p-InGaAsrGaAs system. Combining the Cross-section Transmission Electron Microscopy ŽXTEM. and Deep-level Transient Spectroscopy ŽDLTS. results, we identify that three hole traps and one electron trap H1, H2, H5 and E1 are associated with threading dislocations, and one hole trap H4 is associated with misfit dislocations. We also observed that the dislocations cause degradation in the optical properties of InGaAs and GaAs materials. 2. Experiments To investigate the electrical and optical properties of dislocations, we have designed carefully the composition and thickness of the InGaAs layers. Fig. 1 shows the In mole fraction x and thickness of In xGa 1yx As layers in the four samples grown at a substrate temperature of 5508C and the curve for the In xGa 1yx As layer critical thickness, according to the Dodson–Tsao plastic flow model w12x. The detailed sample structures are show in Table 1. The cap p-GaAs layer in sample 1, 2, 3 is used to stop Be out-diffusion in pq layer.
Table 1 The structures of four samples Sample 1
2
3
4
˚. p GaAs Ž200 A Ž . p-GaAs 1 m m
˚. p GaAs Ž200 A ˚ Ž p-GaAs 2000 A.
˚. p GaAs Ž200 A Ž . p-GaAs 1 m m
˚. p-In 0.05 Ga 0.95 As Ž500 A p-GaAs Ž0.1 m m.
˚. p-In 0.12 Ga 0.88 As Ž500 A p-GaAs Ž0.1 m m.
˚. p-In 0.24Ga 0.76 As Ž650 A p-GaAs Ž0.1 m m.
˚. pq In 0.05 Ga 0.95 As Ž200 A Ž . p-In 0.05 Ga 0.95 As 2 m m p-GaAs Ž0.1 m m.
˚. GaAs Žundoped 200 A nq GaAs Žbuffer 0.5 m m. nq Sub GaAs
˚. GaAs Žundoped 200 A nq GaAs Žbuffer 0.5 m m. nq Sub GaAs
˚. GaAs Žundoped 200 A nq GaAs Žbuffer 0.5 m m. nq Sub GaAs
˚. GaAs Žundoped 200 A nq GaAs Žbuffer 0.5 m m. nq Sub GaAs
q
q
q
The Ž001. nq GaAs substrates are Si doped with doping concentration 1 = 10 18 cmy3 . The nq GaAs buffer layers are Si doped with doping concentration 2 = 10 18 cmy3 . The p-GaAs or p-InGaAs layers are Be doped with doping concentration 1 = 10 17 cmy3 . The pqy GaAs or InGaAs cap layers are Be doped with doping concentration above 2 = 10 18 cmy3 .
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3. Results and discussion 3.1. A. InÕestigation of structure defects and dislocation distribution by TEM Four samples were studied by Cross-sectional Transmission Electron Microscopy ŽXTEM. to investigate the structure defects associated with lattice mismatch. The main structural defects are misfit dislocations and threading dislocations. Most misfit dislocations in the InGaAsrGaAs system are 608 dislocations with four 1r2²110: Burgers vector inclined at 458 to the Ž001. interface plane w13x. In the InGaAsrGaAs system Žepilayer under compression., any 608 dislocation at interface has the extra half plane lying in the GaAs substrate. In this paper, we define the misfit dislocation as the 608 dislocations at the interface. Other dislocations are defined as threading dislocations. 3.2. Sample 1 We performed XTEM on four specimens of sample 1 and found no dislocations in them. Fig. 2a is an XTEM picture of sample 1 and the sample structure is shown in Table 1, sample 1. Since the InGaAs layer in our sample 1 is much thinner than the critical thickness Žsee Fig. 1., misfit dislocations should not be found in the sample. No defects Ždislocation, stacking faults. were observed in the four specimens by TEM even when the micrograph areas were moved around several m m in each specimen. In Fig. 2a, the strain of the interfaces were studied. There are two sharp interfaces of InGaAs layer, but the contrast of the lower InGaAsrGaAs interface is much sharper than the upper interface, and the bend contour in GaAs layer is shifted more at the lower interface than at the upper interface. The above results shows that there are stronger strain at the lower interface than at the upper interface. On the other hand, the strain at the interfaces causes the dark contrast band near the interface. At the lower interface, the dark contrast band as well as the strain are distributed symmetrically on both sides of the interface. However, the dark contrast band and the strain at the upper interface are almost distributed only in GaAs cap layer. 3.3. Sample 2 Three types of dislocations were found in sample 2 and their distributions were also studied. The structure of sample 2 is shown in Table 1, sample 2 and the corresponding XTEM picture of this sample is shown in Fig. 2b. Owing to the GaAsrInGaAsrGaAs sandwich structure of sample 2, there are two misfit dislocation types in this system: the dislocation dipole and the single-dislocation w13,14x. The dislocation dipole is a pair of parallel 608 dislocations with Burgers vectors of opposite sign, lying in
Fig. 2. XTEM micrographs of the four samples shown in Fig. 1. Ža. The bright-field image of sample 1 shows no dislocation in the sample. Žb. The dark-field image of sample 2 shows the dislocations and their distribution in the sample. Žc. The weak-beam image of sample 3 shows the very high density of threading dislocation in InGaAs layer and dislocations in cap GaAs layer in the sample. Žd. The dark-field image of sample 4 shows the dislocations and their distribution in the interface and lower GaAs layer in the sample. The stripes in InGaAs and GaAs layers are the thickness fringes of the specimen.
the same 1114 glide plane, one at each interface. The single-dislocation which is not part of a dipole only appears at lower interface. Fig. 2b shows the two types of misfit dislocations, one is the dislocation dipole, marked as D, and the other is the single-dislocation, marked as S. In Fig. 2b, we also note that there is no dislocation in the cap GaAs layer, however there are some threading dislocations in the lower GaAs layer. According to Lefebvre et al. w15x, segments of reacted misfit dislocations inclining towards the substrate are due to the fact that misfit stress forces are generally higher than the interaction
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elastic force in InGaAsrGaAs system. These segments of misfit dislocations in GaAs layer are defined as threading dislocations in this paper. The threading dislocations only appear in the lower GaAs layer in Fig. 2b and can be explained as follows. All the dislocations at the upper interface belong to dislocation dipoles, however some dislocations at the lower interface are single dislocations. The interaction forces between the pair of dislocation dipoles stop the segments of dislocation inclining towards the upper or lower GaAs layers. Only segments of reacted single dislocations can incline towards the GaAs substrate to form the threading dislocations. 3.4. Sample 3 The structure of sample 3 is shown in Table 1, sample 3. In this sample, the In mole fraction x of In xGa 1yx As layer is about 0.24 and there is a large lattice difference Ž1.7%. between the GaAs layer and the InGaAs layer. With increasing lattice mismatch, the MBE growth mode changes from a two-dimension Ž2D. mode to a three-dimension Ž3D. mode. It was reported by Tabuchi et al. w16x that the 2D growth mode predominates in the In xGa 1yx As layer with In mole fraction x less than 0.5. Ceschin and Massies w17x has shown that the growth undergoes a transition from 2D to 3D if the In composition x is greater than 0.2. The TEM picture of our sample 3 shows that the 2D growth mode dominates in the InGaAs layer, although the In mole fraction x is larger than 0.2. Therefore, our work supports Tabuchi et al.’s conclusion and is in conflict with the work of Ceschin and Massies. The XTEM picture of the sample is shown in Fig. 2c. Fig. 2c is the weak beam dark field image. It shows that in the InGaAs layer there are no stacking faults or micro-twins which are the typical defects in 3D growth epitaxy layer. Therefore, the InGaAs layer is grown with 2D growth mode. Fig. 2c also shows that there is a very high density of threading dislocations in the InGaAs layer. Correspondingly, the quality of the cap GaAs layer, which has many defects, is poor. The stacking faults and threading dislocations are also found in the cap GaAs layer in the XTEM picture. However, the threading dislocation density in the cap GaAs layer is much lower than that in the InGaAs layer. The density of threading dislocations in the InGaAs layer is so high that the reacting dislocations cannot go into the lower GaAs substrate as in sample 2. The lower GaAs layer is dislocation free for sample 3. Fig. 3 is a high-resolution image in the ²110: projection of the InGaAsrGaAs interface of sample 3. Three phenomena were found as below. Ž1. Dislocations are observed at interface in sample 3. These dislocations are threading dislocations shown in Fig. 2c, not the 608 misfit dislocations with regular cross-grid
Fig. 3. High-resolution TEM image of sample 3 shows two dislocations with the extra half plane lying in GaAs layer.
configurations that appeared in sample 2. However, the projected Burgers vector b of the dislocations in sample 3 is the same as the 608 misfit dislocation’s Burgers vector b. Ž2. Islands which are usually observed at 3D growth mode interface are not found in the image and in the InGaAs layer which indicate that there are no stacking faults and micro-twins which are the typical defects in 3D growth mode layer. Fig. 3 shows again that a 2D layer-bylayer growth mode dominates in InGaAs layer. Ž3. Two dislocations with the same Burgers vector b are found and they are so close to each other that the distance ˚ This indicates that the density between them is only 50 A. of threading dislocations in the InGaAs layer is very high. 3.5. Sample 4 The structure of sample 4 is shown in Table 1, sample 4 and the XTEM picture is shown in Fig. 2d. In this sample, no dislocations can be observed in the InGaAs layer except at the interface. The misfit dislocations are located in the interface, similar to the single dislocations in sample 2. The segments of interaction dislocations incline towards the GaAs substrate to form threading dislocations. In this sample, there are some threading dislocations in the GaAs buffer layer which are indicated by arrows in the picture. These threading dislocations propagate into GaAs layer as deep as 0.9 m m below the InGaAsrGaAs interface. In summary, the XTEM pictures of the above four samples show the following.k Sample 1 is dislocation free, but there are strains in the both InGaAsrGaAs interfaces. In samples 2 and 4, there are 608 misfit dislocations. There are some single dislocations along the lower InGaAsrGaAs interface and there are some threading dislocations inclining toward the GaAs substrate. The InGaAs layer in sample 4 is dislocation free. For the sandwich structure in sample 2, there are also dislocation dipole pairs along the upper and lower InGaAsrGaAs interface.
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The dislocation dipoles do not induce any threading dislocations inclining toward the GaAs layer. Therefore, the cap GaAs layer in sample 2 is dislocation free. For sample 3, although the lattice mismatch between In 0.24 Ga 0.76 As and GaAs is as large as 1.7%, the growth mode in InGaAs layer is still two-dimensional. There is a high density of threading dislocations in InGaAs layer. Correspondingly, the quality of the cap GaAs layer is poor, with many defects Žstacking faults and threading dislocations. in the cap layer. No misfit dislocation is found in sample 3. The GaAs substrate is free of dislocations. 3.6. InÕestigation of traps by DLTS measurements Four kinds of diode are used in DLTS measurements. The diodes are made from the same sample structures which have been studied by XTEM, as indicated in Table 1. All diodes used in DLTS measurement have the same area Ž0.1257 mm2 . of p–n junction. As is well known in DLTS measurement, by using different reverse bias voltage Vr and filling pulse height V h , we can detect the trap signal contributed from traps located in different depth regions w18x. When the edge region effect of the deep trap level is considered w19x, the actual active DLTS region is slightly modified and extended deeper to the depletion region. By using different bias and filling pulse voltage, we have different active DLTS region and therefore we can distinguish different traps located in different layers. By this method, we can correlate the relationship between different DLTS trap signals and the corresponding structure defects found in XTEM studies. Fig. 4 shows some typical DLTS spectra of samples 1–4. Six hole traps H1, H2, H3, H4, H5, H1X and two electron traps E1, E2 are found in these samples. Fig. 5 shows the corresponding Arrhenius plots of hole traps DLTS measurements. The hole trap activation energies are calculated from the Arrhenius plot and are listed in Table 2. H1 and H2 hole traps: We identify H1 and H2 as hole traps associated with threading dislocations. This is based on the following facts: Ž1. H1 and H2 hole traps have very large DLTS signal in sample 3, comparatively weak signal in sample 4, very weak signal in 2 and no signal in sample 1. Ž2. For sample 3, when the DLTS active region includes the InGaAs region, the H1 and H2 DLTS signal is very large. When the active DLTS region moves up to the GaAs cap region only, the DLTS signal gradually decreases. Ž3. For sample 4, when the active DLTS region includes the lower GaAs region, we can detect the H1 and H2 DLTS signal. When the active DLTS region moves up and only include the InGaAs region, the H1, H2 DLTS signal disappears. Ž4. For sample 2, when the active DLTS region includes the lower GaAs layer, the H2 DLTS peak is very small with a very weak tail at the low temperature side, probably contributed from H1 traps. When the active
Fig. 4. DLTS spectra of four samples with a rate window 200 sy1 ; the solid curves are measured at small reverse bias with large forward bias injection condition. The dotted curves are the typical DLTS spectra without forward bias injection condition. Ža. Sample 1, there is no any DLTS signal when forward bias injection is not applied. Žb. Sample 2, When the active DLTS region move up to the cap GaAs layer, no any DLTS signal is detected. Žc. Sample 3, when the active region of DLTS move up to the cap GaAs layer, the H5 DLTS signal becomes larger. Žd. Sample 4, when the DLTS active region move up to the InGaAs layer, no any DLTS signal is detected. The inset shows DLTS peak height as function of filling pulse duration for hole traps H2 and H4.
DLTS region moves up to exclude the lower GaAs layer, the H2 and H1 trap signal completely disappears. These DLTS results are completely consistent with the threading dislocation distribution detected by XTEM. Hole traps similar to H1 and H2 have been reported by Ashizawa et.al. w6x Ž0.31 eV. and Choi w7x Ž0.16–0.40 eV. that they are associated with dislocations in the lattice-mismatched interface. H4 hole trap: We attribute H4 hole trap to single and dipole misfit dislocations along the InGaAsrGaAs interface. This is based on the following facts. Ž1. H4 DLTS peak only appears in 2 and 4 samples and disappears in 1 and 3 samples. Ž2. For sample 2, H4 DLTS signal only appears when the active DLTS region includes the InGaAs layer and the lower GaAs p-GaAs layer. When the active DLTS region moves up to only includes the cap GaAs region, the H4 DLTS peak disappears. Ž3.
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Fig. 5. Arrhenius plots for observed electron and hole traps in four p-In x Ga 1yx AsrGaAs heterostructure samples.
For sample 4, H4 DLTS signal only appears when the active DLTS region includes the InGaAsrGaAs interface. These DLTS results are completely consistent with the misfit dislocation Žincluding the dislocation dipole and single dislocation. distribution detected by XTEM. H5 hole trap: We identify H5 as another trap associated with threading dislocation in GaAs. This is based on the following facts: Ž1. H5 appears in sample 3. When the active DLTS region moves up to the cap GaAs layer, H5 DLTS signal increases. H5 also appears in sample 4. However, when the active DLTS region moves up and excludes the lower GaAs layer, the H5 signal disappears. Ž2. There is no H5 DLTS signal for sample 1. The DLTS results are consistent with the XTEM picture of threading dislocations. H3 hole trap: H3 is only found in sample 4 when the active DLTS region is in the InGaAs layer. According to sample 4 XTEM picture, the InGaAs layer is free of dislocations. Therefore, H3 hole trap is not likely to be induced by dislocation and its origin is not clear. H1X hole trap: Only found in sample 1 under a very large forward bias injection. Since sample 1 is free of dislocations, H1X is not related to any dislocations. H1X is also not related to InGaAsrGaAs interface, otherwise, it should be detected in other samples too. The origin of H1X hole trap is not clear yet.
E1 electron trap: E1 DLTS signal was only detected in sample 3 under a very large forward bias injection. We tentatively assign this E1 electron trap to be threading dislocation trap, since only sample 3 has high density of threading dislocations Žsee Fig. 2c.. Electron trap similar to E1 has been assigned as a dislocation trap by many authors w8–10x. E2 electron trap: E2 DLTS signal was detected in all samples Ž1, 2, 3, 4., and only appears under the condition of a very large forward bias injection. We note that E2 also appears in sample 1 which is dislocation free, therefore E2 is definitely not induced by dislocations. Another interesting fact is that the DLTS peak temperature of E2 trap is almost unchanged with rate window and the corresponding activation energy of E2 is larger than the energy gap of GaAs. Therefore, E2 energy level is located in the valance band as a resonant level. We tentatively assign the E2 electron trap to be an interface state of GaAsrInGaAs heterojunction. Further study is needed to clarify this E2 electron trap. Finally, we have checked the DLTS peak amplitudes with the DLTS filling pulse duration time t p . The experimental results for the well resolved H2 and H4 DLTS peaks are plotted in the inset of Fig. 4. As indicated in the inset, H2 and H4 peak amplitudes depend on the logarithmic t p . These results are in agreement with previous reports for the dislocation associated traps in GaAs w20,21x and in Si w22x, further supporting our identification in this work. On the contrary, the well resolved DLTS peak amplitude of H1Y remains unchanged within the experimental accuracy when t p changed from 10y4 s to 10y1 s. This also supports that the H1Y trap is not related to any dislocations. In summary, using different bias voltage and pulse height in DLTS measurement, we can detect and distinguish the trap states located in different layers in the samples. Comparing these DLTS information with XTEM pictures, we assign: Three hole trap states H1, H2, H5 associated with threading dislocations. One hole trap state H4 associates with misfit dislocations alone the InGaAsrGaAs interface. One electron trap E1 associates with threading dislocations. One electron trap E2 associates with InGaAsrGaAs interface state. Two hole traps H3 and H1X which are not
Table 2 Hole and electron traps characteristics Trap type X
H1 H1 H2 H4 H5 E1 E2
DLTS activation energy ŽeV.
Identification
0.20 0.32 0.38–0.40 0.67–0.73 0.88 0.54
threading dislocation threading dislocation misfit dislocation threading dislocation threading dislocation
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related to dislocations and the origin is not clear. All the hole and electron trap energy levels are listed in Table 2. 3.7. InÕestigation of dislocation effects by photoluminescence (PL) The low temperature photoluminescence technique is also used to study the effects of dislocation in the four samples. All PL measurements were carried out at sample temperature of 4 K. Fig. 6 shows the PL spectra of samples 1–4, obtained under excitation of Arq laser with power I s 50 mW. We assigned the emission peaks A, B around 1.512 eV and 1.495 eV to band-edge and band-Be acceptor transitions in p-GaAs layer, respectively. The peak D is associated with near band-band transitions in InGaAs layer in different samples. The PL peak positions of InGaAs layers are used to determine the compositions of InGaAs layers in different samples which agree with the results obtained from the Double-crystal X-ray diffraction meter. By comparing the PL spectra of the samples 1–3, it can be seen that the PL intensity is closely related to the dislocation density. The higher the dislocation density in GaAs or InGaAs layer, the lower the PL intensity of GaAs or InGaAs layer. If all Fig. 7. PL spectra at 4 K for sample 1 Ža. and sample 2 Žb. under ˚ Ti–sapphire Laser with different power. excitation of 7800 A
˚ Fig. 6. PL spectra at 4 K for four samples under excitation of 4880 A Arq laser with power I s 50 mW.
the dislocation induced traps are assumed to be non-radiative recombination centers, the PL results are consistent with XTEM and DLTS results. ˚ Further PL explanation should consider that the 500 A InGaAs layer has quantum confinement effects in the 1–3 samples. First, there are high densities of surface states at the GaAs top surface exposed to the air. The PL spectra may be influenced by these surface states. However, due to the quantum confinement effect, the excited electron hole pairs diffuse to and are confined in the quantum well. Therefore, the effect of GaAs surface states are reduced and can be neglected. Second, the PL peak D1 of the InGaAs layer in sample 1, due to the quantum confinement effect, is much higher and narrower than D4 peak of the InGaAs layer in the sample 4 which has 2 m m thickness and almost dislocation-free InGaAs layer. Fig. 7 shows the respective PL spectra of sample 1 and 2, under different excitation power. In Fig. 7a, there are five peaks in sample 1 PL spectra. Peak A1 and B1 are band-edge and band-Be acceptor transitions in p-GaAs layer, peak D1 and E1 are associated with band-edge and band-Be acceptor transitions in InGaAs quantum well. Peak C1 position is strongly dependent of excitation light intensity. Further investigation of peak C1 by photo-luminescence excitation ŽPLE. w23,24x shows that peak C1 is due to the electron transition from the conduction band in the InGaAs to valance band in GaAs near the heterojunction interface. This will be
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reported in another paper in detail. In PL measurement, the quality of the layer is usually characterized by the full-width of half maximum ŽFWHM. of the peak. The FWHM of peaks A1, B1 and D1 are 6 meV at excitation power of 1.0 mW, indicating that the qualities of the InGaAs and GaAs epitaxial layer in sample 1 are good. Fig. 7b shows the PL spectra of sample 2 at different laser power excitations. There are only three peaks A2, B2 and D2 and the positions of all peaks do not change with laser power. Peak D2 ŽInGaAs layer. is so broad that the peak E Žin Fig. 7a. cannot be resolved in the spectra of the sample 2. The energy gap difference between InGaAs and GaAs in sample 2 is larger than that in sample 1, however the InGaAs layer thicknesses in both samples are the same. Therefore, the quantum confinement effect in sample 2 should be stronger than that in sample 1. Although the PL and PLE results show a quantum confinement effect in sample 2, its PL peak intensity of the InGaAs layer is weaker than that in sample 1. The existence of misfit dislocations in InGaAsrGaAs interfaces is the main reason for the degradation of the optical quality of the InGaAs layer in sample 2. The FWHM of the cap GaAs layer A and B are 7.0 meV and 7.0 meV, respectively at excitation power of 1.0 mW and the FWHM of D peak in InGaAs layer is 15.3 meV in sample 2.
4. Conclusions MBE grown p-In xGa 1yx AsrGaAs lattice-mismatched heterostructures with different In mole fraction x and different layer thicknesses are investigated systematically. The misfit dislocations and the threading dislocations observed by XTEM in different samples generally satisfy the Dodson–Tsao’s plastic flow critical layer thickness curve. The XTEM, DLTS and PL results are consistent with each other. By comparing these results, we identify that the threading dislocations in bulk layers introduce three hole traps H1, H2 and H5 with DLTS activation energies of 0.32 eV, 0.40 eV and 0.88 eV, respectively, and one electron trap E1 with DLTS activation energy of 0.54 eV. The misfit dislocations in relaxed InGaAsrGaAs interface induce a hole trap H4 with DLTS activation energy between 0.67–0.73 eV. All dislocation induced traps are
non-radiative recombination centers which greatly degrade the optical property of the InGaAsrGaAs layers. Acknowledgements This work was supported by Singapore National Science and Technology Board RIC-university research project No. 681305. A.Y. Du thanks Prof. K.H. Kuo for the arrangement of TEM work at BLEM. References w1x J.J. Rosenberg, M. Benlamri, P.D. Kirchner, J.M. Woodall, G.D. Pettit, IEEE Electron Dev. Lett. EDL 6 Ž1985. 491. w2x H. Ito, J.S.H. Jun, Electron. Lett. 28 Ž1992. 655. w3x R.J. Fu, C.S. Hong, E.Y. Chan, D.J. Booher, L. Figueroa, IEEE photo. Technol. Lett. 3 Ž1991. 308. w4x R. People, J.C. Bean, Appl. Lett. 47 Ž1985. 322. w5x L.P. Ramberg, P.M. Enquist, Y.K. Chen, F.E. Najjar, L.F. Eastman, E.A. Fitzgerald, K. Kavanagh, J. Appl. Phys. 61 Ž1987. 1234. w6x Y. Ashizawa, S. Akbar, W.J. Schaff, L.F. Eastman, E.A. Fitzgerald, D.G. Ast, J. Appl. Phys. 64 Ž1988. 4065. w7x Y.W. Choi, K. Xie, H.M. Kim, C.R. Wie, J. Electron. Mater. 20 Ž1991. 545. w8x A.C. Irvine, L.K. Howard, D.W. Palmer, Mat. Sci. Forum 83 Ž8. Ž1992. 1291. w9x W.R. Buchwald, J.H. Zhao, M. Harmatz, E.H. Poindexter, Solid-St. Electron. 36 Ž1993. 1077. w10x Y. Uchida, H. Kakibayashi, S. Goto, J. Appl. Phys. 74 Ž1993. 6720. w11x C. Herbeaux, J. Di Persio, A. Lefebvre, Appl. Phys. Lett. 54 Ž1989. 1004. w12x B.W. Dodson, J.Y. Tsao, Appl. Phys. Lett. 51 Ž1987. 1325. w13x T.J. Gosling, R. Bullough, S.C. Jain, J.R. Willis, J. Appl. Phys. 73 Ž1993. 8267. w14x J. Zou, D.J.H. Cockayne, B.F. Usher, J. Appl. Phys. 73 Ž1993. 619. w15x A. Lefebvre, C. Herbeaux, J. Di Persio, Phil. Mag. 63 Ž1991. 471. w16x M. Tabuchi, S. Noda, A. Sasaki, J. Cryst. Growth 99 Ž1990. 315. w17x A.M. Ceschin, J. Massies, J. Cryst. Growth 114 Ž1991. 683. w18x M.F. Li, in: Modern Semiconductor Quantum Physics, ŽWorld Scientific, Singapore, 1994. p. 360. w19x M.F. Li, C.T. Sah, IEEE Trans. Electron Devices ED-29 Ž1982. 306. w20x T. Wosinski, J. Appl. Phys. 65 Ž1989. 1566. w21x G.P. Watson, D.G. Ast, T.J. Anderson, B. Pathangey, Y. Hayakawa, J. Appl. Phys. 71 Ž1992. 3399. w22x P. Omling, E.R. Weber, L. Montelius, H. Alexander, J. Michel, Phys. Rev. B32 Ž1985. 6571. w23x E. Cohen, M.D. Sturge, Phys. Rev. B15 Ž1977. 1039. w24x M.F. Li, In: Modern Semiconductor Quantum Physics, ŽWorld Scientific, Singapore, 1994. p. 189.