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GaAs heterojunction bipolar transistor optical and electronic band structure characterization
Thin Solid Films 364 (2000) 58±63 www.elsevier.com/locate/tsf
InGaP/GaAs heterojunction bipolar transistor optical and electronic band structure characterization M. Murtagh a,*, J.T. Beechinor a, N. Cordero a, P.V. Kelly a, G.M. Crean a, I.L. Farrell b, G.M. O'Connor b, S.W. Bland c a National Microelectronics Research Centre, Lee Maltings, Prospect Row, Cork, Ireland National Centre for Laser Applications, Department of Experimental Physics, University College Galway, Ireland c Epitaxial Products International Ltd., Pascal Close, Cypress Drive, St. Mellons, Cardiff, South Glamorgan CF3 OEG, Wales, UK b
Abstract In this work we investigate the optical and band structure properties of full InGaP/GaAs based heterojunction bipolar transistor (HBT) epitaxial structures grown by metalorganic chemical vapour phase epitaxy (MOVPE). In related work, full HBTs have been fabricated from the two wafers studied, which exhibit high and low common-emitter current gain (hFE) parameters on electrical test. The focus of this study is to investigate and compare the photoluminescence and photore¯ectance spectroscopy response of these known good and bad epitaxial wafers. The results of low temperature (10±300 K) spectral and transient photoluminescence (PL) analysis are presented, revealing evidence of the nature of the InGaP ordering induced non-radiative loss mechanism. The results also demonstrate the modi®cation to the PL lineshape arising from the InGaP/GaAs interfacial conditions. The experimental results are supported by X-ray diffraction data and ®nite-element device simulation, showing the effect of intermixing layers on the interfacial band potentials. The optical modulation technique of photore¯ectance (PR) spectroscopy was employed to investigate the band structure and interfacial electric ®elds, Fs, of the HBT structures. Following the polarisation±[110] and [11Å0]±dependence of the sub-lattice ordering PR response, it was found necessary to include an emitter/ base intermixing layer in order to account for the InGaP/GaAs Fs data. It is concluded that non-optimal MOVPE growth conditions for one of the structures resulted in both sub-lattice ordering and layer intermixing effects, consistent with the low hFE of the HBTs fabricated from this material. q 2000 Elsevier Science S.A. All rights reserved. Keywords: InGaP/GaAs HBT; Ordering; Intermixing; Photoluminescence; Photore¯ectance
1. Introduction The unique heterojunction properties such as large bandgap, favourable band discontinuities and low interface recombination velocity make the InGaP/GaAs system an attractive candidate for a variety of electronic and optical devices such as heterojunciton bipolar transistors (HBTs) [1]. Moreover it is not easily oxidised, has a high etch selectivity with GaAs when compared to AlGaAs and unlike the latter, no deep level traps, e.g. DX centres have been thus-far reported. Importantly however, is the occurrence of spontaneous `CuPt' type long range ordering in the ternary alloy i.e. tendency of the InGaP cations to order on the group III sub-lattice, producing alternate In and Ga rich (11Å1) and (1Å11) planes [2], with consequent zone folding (L point and zone centre), resulting in a reduced interband
* Corresponding author. E-mail address: [email protected] (M. Murtagh)
transition energy within the emitter layers, as well as polarised (i.e. directional dependent) optical properties [2]. During the growth process itself, attainment of perfectly abrupt InGaP/GaAs interfaces is dif®cult, leading to interfacial mixing as a result of growth precursor `memory' effects, as well as other growth parameter related lattice matching issues including temperature and alloy composition. Both InGaP ordering and InGaP/GaAs layer interfacial mixing may lead to poor HBT devices. Ordering may reduce the carrier injection while interfacial mixing may lead to additional interfacial carrier recombination mechanisms, i.e. where the p±n and metallurgical junctions are not coincident, hence resulting in reduced HBT current gain [3]. Note that under most operating conditions HBT degradation is predominantly current driven [4]. Note also that interfacial mechanical stress arising from the presence of mixing layers, has important implications for electrical property degradation of the HBT device, proving to be a catalyst for defect creation and migration [4].
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0090 2-5
M. Murtagh et al. / Thin Solid Films 364 (2000) 58±63
In this work, we report the results of both a photoluminescence (PL) and a photore¯ectance (PR) optical spectroscopic study of the electronic band structure for two representative HBT structures, showing both good and bad electrical device operation. The spectroscopic results are consistent with the occurrence of both ordering and interfacial mixing for the bad HBT, and demonstrate in particular the importance of inclusion of both effects for a full understanding of the data. 2. Experimental and results The GaAs/InGaP based HBT samples in this study were grown on (100) 28 off-axis toward (110) GaAs substrates using MOVPE, with the following structure: collector contact layer of n 1 GaAs followed by n-GaAs collector, a base layer of p 1 GaAs with a similar thickness n-InGaP emitter, n 1 InGaP emitter contact and ®nally a thick n 1 GaAs cap. Two sample sets were examined, one exhibiting good device performance, the other revealing non-optimal MOVPE growth and poor HBT device characteristics. PL measurements were recorded in a closed cycle cryostat (10 to 200 K), with Ar 1 (488 nm) laser excitation and CCD detection. Transient PL data were recorded using a photon counting PMT detector (5 ns pulse width), with 10 kHz pulsed laser excitation using acoustic-optic modulation, with a matched 5 ns fall time. PR spectroscopy was performed at room temperature (RT) using a twin scanning arrangement [2] over the 1.3± 2.l eV range, with excitation from an Ar 1 (488 nm) laser source. Previous double crystal X-ray diffraction data [5] indicated the presence of InGaP/GaAs interfacial mixing, i.e. between the GaAs cap and InGaP emitter as well as between
59
the emitter and the p 1 GaAs base layer for the poor HBT device. The data in particular revealed an approximate 2 nm In0.38Ga0.62P intermixing layer at the emitter/base interface, correlating with device electrical results showing a variation in the common emitter current gain, hFE, equal to 11 for the poor HBT, compared to a value of 79 for the good HBT device. The results of the poor HBT structure are furthermore consistent with the measured emitter/base current-voltage data, exhibiting large ideality factors, i.e. consistent with non-ideal thermionic emission [6] and possibly arising from a stress induced increase in the carrier recombination [6], but also indicating a possible barrier height voltage dependence, due to the existence of a thin interfacial layer [6]. Fig. 1 presents the PL spectral evolution of both HBT InGaP layer structures as the temperature increases from 10 to 200 K. Two well resolved peaks are observed for both, with a stronger overall luminescent signal recorded for the poor HBT±approximately twice the PL intensity of the nominal HBT at 10 K, and con®rmed by RT measurements. The spectral data are summarised in Table 1, with a full width half maximum (FWHM) of 16 meV recorded for the high energy peak (10 K) for the good HBT, and indicative of good quality material. Arrhenius analysis of the normalised integrated PL intensities yield values for the activation energies of any nonradiative loss mechanism(s), according to [7] R T 1 1A Ce 2EA =kT 1B Ce 2EB =kT 21
1
where EA and EB are the thermal activation energies and AC and BC are effectively the ratio of non-radiative to radiative recombination probabilities for the two loss mechanisms at RT.
Fig. 1. Spectral evolution of PL peaks with temperature for both HBT InGaP emitter layers.
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M. Murtagh et al. / Thin Solid Films 364 (2000) 58±63
Table 1 Summary of LT PL, PR and ®nite-element device model analysis showing the effect of inclusion of intermixing layers upon the electric ®eld and lineshape data a Photoluminescence (10 K) Nominal HBT 1.961 eV peak EA 11.1 meV A C 12.8 EB 62.9 meV B C 499 1.918 eV peak EA 35.5 meV A C 7205 EB 9.53 meV B C 9.41 a
Photore¯ectance (RT) `Poor' HBT
1.89 eV peak 17.7 meV 35.9 60.9 meV 648
Finite-element model
Nominal HBT
HBT `Poor'
Nominal HBT
`Mixing' HBT
E0(InGaP) 1.905eV
1.853 eV
G 8.99 meV
7.87 meV
FC 26 kV/cm
23 kV/cm
FC 26 kV/cm
26 kV/cm
FE 111 kV/cm
108 kV/cm
FE 110 kV/cm
133 kV/cm
The PL (Arrhenius) non-radiative loss mechanisms are summarised along with the corresponding probabilities ( A/BC).
Fig. 2 shows the result of a least-squares regression to the above R(T) equation for the good HBT structure, i.e. for the high energy peak and assuming two activation energies, with results summarised in Table 1. The plot moreover could not be ®t with just one non-radiative mechanism. From Table 1 the dominant activation energy for this peak is 63 meV while for the second lower energy 1.918 eV PL peak, a dominant non-radiative loss mechanism with an energy of around 35.5 meV was observed. The broader low energy peak is rapidly quenched by temperature, moving from 1.918 to 1.896 eV over the temperature range 10 to 70 K. Its energy shift does not follow that of the high energy peak, thus eliminating the possibility of a phonon replica. The spectral evolution for the non-optimal HBT again
reveals a strong low energy component, Fig. 1b, however as the temperature increased the peaks were poorly distinguished due to the spectral broadening of the low peak. The energies of the peaks were established at 1.868 and 1.89 eV, considerably lower then that of the nominal HBT structure and indicative of atomic ordering [7]. The low energy PL furthermore is quenched by 60 K, while the other peak initially blueshifts up to a temperature of about 70 K whereupon it begins to redshift, a behaviour characteristic of ordered material. That is the `turn-around' temperature depends upon the amount of energy required to thermalize the electrons/holes out of the ordered induced band-edge ¯uctuations, into those band-edge states of the randomly ordered crystalline matrix [8]. As a consequence of the spectral broadening of the low
Fig. 2. Arrhenius thermal behaviour of the normalised PL peak data for both HBT InGaP layers, showing the results of a least squares regression to the R(T) equation.
M. Murtagh et al. / Thin Solid Films 364 (2000) 58±63
energy component, Arrhenius data were determined for this sample only after removing data from 20 to 60 K (inclusive) and by plotting the intensities at 10 K and from 70 to 300 K, i.e. from which good ®ts were obtained. As shown in Table 1, the dominant activation energy for non-radiative loss was 61 meV. The higher activation energies of around 60 meV extracted from the high PL transitions of both HBTs have been suggested to be indicative of a deep level trap, arising from possible deep acceptor, neutral impurity or even a vacancy related defect [7]. The most likely cause for the lower activation energies varying from 9.5 to 17.7 meV, has also been speculated to be characteristic of trapped excitons or carriers thermalising from localised regions of ordered induced band-edge ¯uctuations, followed by nonradiative recombination [7]. Interestingly for the good HBT however, when combining both loss energies for the low 1.918 eV peak, the resulting energy (35.5 1 9.5) 45 meV, closely resembles the difference in energy separation of the two PL peaks in question, 43 meV. This is therefore suggestive of the nature of the ordering induced loss mechanism, namely that energy required for the trapped excitons or carriers to thermalise and diffuse to the vicinity of the disordered higher energy band-edge minima, 1.961 eV. The relative activation intensity ratios should be noted however for the two mechanisms, revealing the relative dominance of EA ( AC). However it should also be noted that the ratios AC and BC are not uniquely determined, with the radiative transition probability in general expected to vary as a function of temperature [7]. As shown in Fig. 3, time resolved PL measurements with lifetimes de®ned as the decay interval to 1/e of peak value, were investigated for both HBT InGaP layers. The lifetime
61
of the high energy peak for the good HBT was measured to be 26 ^ 3 ns at an excitation power of 5 kW/m 2. This short decay is consistent with excitonic recombination, as well as with both the observed laser excitation and power spectrum evolution trends. The longer lifetime of 100 ^ 10 ns (7.9 kW/m 2) recorded for the low energy peak is also consistent with some level of spatial carrier separation arising from possible isolation of impurity centres, as indeed indicated by laser excitation experiments. Longer lifetimes of 34 ^ 3 ns at both 7.9 kW/m 2 and 260 W/m 2 excitation levels were recorded for the poor HBT InGaP layers, Fig. 3, again con®rming the ordered nature of the alloy layers. Note that low incident excitation powers (260 W/m 2) were necessary in order to discern the longer lifetimes expected from the ordered sample, i.e. as the ®nite number of states in the ordered domains saturate ± before the faster component due to recombination in the randomly ordered matrix begins to dominate. It is important ®nally, to note the higher luminescent intensity observed for the poor HBT sample, notwithstanding previous studies detailing reduced PL intensities for ordered InGaP [7,8]. The PL response, i.e. both LT and RT band edge data moreover ± assuming no signi®cant dopant variation between samples, suggest some modi®cation from the InGaP/GaAs interfacial conditions. That is consistent with the previously observed intermixing n 1 GaAs/emitter/p 1 GaAs layers, resulting in a possible reduction in the number of interfacial non-radiative recombination states and hence increased PL intensity. Other ordering induced Fermi level pinning effects however may also account for the PL intensity variation, as will be discussed later during the photore¯ectance data. The experimental results are correlated with ®nite element device modelling, i.e. self-consistent Poisson and
Fig. 3. Transient PL response for both HBT emitter layers - the laser intensities are also shown.
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M. Murtagh et al. / Thin Solid Films 364 (2000) 58±63
electron-hole transport solver [9] of space distributed values for the HBT structures. The simulations were performed assuming an InGaP/GaAs conduction band offset of 0.20 eV [1]. As detailed in Table 1, the model results, which includes estimates of photovoltage effects upon the electric ®elds [3], show that both InGaP/GaAs interfacial ®elds increase following the inclusion of interfacial, i.e. previous DCXRD intermixing layers, between both the GaAs cap and InGaP emitter as well as between the emitter and p 1 GaAs base. Speci®cally for the poor HBT, an increase of 23 kV/ cm is observed for the electric ®eld in the base/emitter interface region. Fig. 4 presents the recorded photore¯ectance (PR) data for both HBT structures, with band structure responses from both the GaAs (1.42 eV) and InGaP (<1.9 eV), i.e. collector and emitter HBT regions, respectively. The spectra exhibit typical medium to high surface electric ®eld response as indicated by the exponentially broadened band-edge transition, E0, along with above band-gap Franz±Keldysh oscillations (FKO) [3]. From the FKO period, estimates of the electric ®elds may be extracted, i.e. assuming effective GaAs electron and heavy hole masses of 0.067 and 0.34(001) while for InGaP, masses of 0.118 and 0.66(001), respectively, were employed [10]. The results of a Newton±Gauss least squares regression analysis of a medium ®eld functional form (Airy functions) [3] to the InGaP experimental data are also shown (- - -) in Fig. 4, with extracted spectral parameters, i.e. electric ®eld, Fs, E0 and phenomenological lifetime broadening parameters, G all summarised in Table 1. It should be noted that the in¯uence of slowly varying low-®eld PR structure from highly doped layers was not taken into account in the model. Small reductions in the collector/base ®eld between both HBT's may be a result of lower collector doping for the poor performance HBT, however for the InGaP PR structure, i.e. the base/emitter interface, reductions in both the
transition energy and electric ®eld data are observed. The recorded band-gap shrinkage of about 50 meV is consistent with the PL data, viz. revealing self-ordering effects. The degree of self-ordering may be determined by [8]
h2
E0 h0 2 E0 h h1 DEBGR
2
where E0 h0 1:915 eV and DEBGR h1 0:471 eV [8] and revealing a h value of approximately 0.36 for the poor HBT structure. In order to con®rm this ordering induced band-gap shrinkage however, i.e. not variations in alloy mole fraction, InGaP PR spectra were also recorded for incident light polarised along both [110] and [11Å0] directions. As shown in Fig. 4b, the observed red shift of the [110] polarisation can be used to estimate the degree of ordering, h , i.e. with reference to previous data [8] and determined also to be approximately 35%, corresponding to a polarisation shift of E0110 2 E0110 < 8±10 meV. Thus from the ordering induced piezoelectric effect [8], an estimate of the associated ®eld shrinkage is 25±30 kV/cm. It should be noted however that this ®eld reduction assumes ordering of undoped layers and does not account for possible carrier screening effects from the doped InGaP. Hence when accounting for both an intermixing layer increase in the interfacial ®eld (23 kV/cm), as well as an ordering (piezoelectric) ®eld reduction, a value of 103±108 kV/cm is therefore determined for the emitter/base ®eld, in reasonable agreement with the PR experimental data, 108 kV/cm, Table 1. The level of ordering induced valence-band-splitting (VBS) may be determined from a calculation involving the valence band Hamiltonian [11], and determined for the poor HBT to be 14.9 meV, assuming a crystal ®eld splitting of 0.2 eV and a spin orbit splitting of 0.1 eV [11]. Interest-
Fig. 4. (a) PR spectra for both GaAs collector and InGaP emitter regions for both HBT structures. Best ®t analysis (- - -) are also shown for the emitter PR structure. (b) Spectral shift observed for the polarized PR InGaP response from the ordered HBT.
M. Murtagh et al. / Thin Solid Films 364 (2000) 58±63
ingly therefore unlike previously reported, both the VBS and measured polarisation shift of 8±10 meV differ from each other by more than a factor of two [11], consistent with some level of InGaP/GaAs interfacial strain, i.e. some intermixing in¯uence upon the VBS, as measured from the XRD data. As well as the potential carrier screening effects of the piezoelectric ®eld for doped InGaP, it must also be mentioned that the above results do not take into account other possible ordering induced changes, such as the transition from a type I to a type II band alignment for fully ordered-InGaP/GaAs interfaces [8]. The analysis also assumes little or no ordering induced changes to the effective band masses, m *, i.e. as a result of two competing mechanisms, viz. reduced band-gap (decreased m *) and G ±L band states coupling (increased m *) [8]. The in¯uence of ordering upon m * however is assumed to be negligible, especially given the square root dependence with ®eld [3]. Moreover and most signi®cantly, a theoretical treatment has even suggested that the interface between a large domain of ordered InGaP and a cubic material like GaAs will very likely have the Fermi level (EF) pinned at one of the band edges [8], with the possible excess electrons or holes signi®cantly modifying the offset. This could alternatively explain the apparently anomalously reduced electric ®eld for the poor HBT, i.e. assuming EF pinning near the conduction band at the emitter/base interface, hence leading to an increase in the emitter/base interfacial ®eld. This would also have implications for the previous PL intensity data, that is with conduction band-edge EF pinning, populating possible interfacial non-radiative states, thus leading to a decrease in the ratio of non-radiative to radiative recombination, and hence increased PL intensity, R. 3. Conclusions The photoluminescence (PL) and photore¯ectance (PR) spectral responses of InGaP/GaAs HBT structures of known good and bad device performance (current gain hFE) were compared. The Arrhenius PL study reveals evidence for the
63
nature of the InGaP ordering induced non-radiative loss mechanism, while the PR ®eld data was interpreted by including InGaP/GaAs layer intermixing. The results demonstrate in particular the effect of sub-lattice ordering as well as interfacial mixing layers upon both the PL and PR spectral lineshape responses. Further issues relating to the band structural electronic response, including ordering induced Fermi level pinning, were also outlined. It is concluded that non-optimal MOVPE growth conditions for one of the structures resulted in both sub-lattice ordering and layer intermixing effects, consistent with the low hFE of the HBTs fabricated from this material. Further work will be required to elucidate the exact mechanisms in order to correlate the results with those of the electrical device data, as well as to investigate the relative dominance of ordering or intermixing upon HBT device performance. Acknowledgements Brite-EuRam project: HEROS, contract number BRPRCT98-0789. References [1] H. Kroemer, J. Vac. Sci. Technol. B. 1 (1983) 126. [2] E. Gregor, K.H. Gulden, M. Moser, G. Schmiedal, P. Kiesel, G.H. Dohler, Appl. Phys. Lett. 70 (11) (1997) 1459. [3] F.H. Pollak, W. Krystek, M. Leibovitch, H. Qiang, Am. Inst. Phys. (1996) 669. [4] T. Henderson, W.L. Chen, M. Sanna, Future Fab, Int. (1997) 229. [5] E. Richter, F. Brunner, S. Gramlich, S. Hahle, M. Mai, U. Ziemer, M. Weyers, Invited Paper, Symp. 5, EXMATEC `98 4th Int. Workshop on Cont. of Comp. Semi. Mater. Technol., 21st±24th June, Cardiff, Wales. [6] K. McCarthy, C. Lyden, W.M. Kelly, Solid-State Electron. 34 (2) (1991) 220. [7] J.D. Lambkin, L. Considine, S. Walsh, G.M. O'Connor, C.J. McDonagh, T.J. Glynn, et al., Appl. Phys. Lett. 65 (1) (1994). [8] S. Froyen, A. Zunger, A. Mascarenhas, Appl. Phys. Lett. 68 (1996) 2852. [9] Silvaco Data Systems: Santa Clara, CA. [10] Y. Zhang, A. Mascarenhas, Phys. Rev. B 51 (1995) 13162. [11] S.H. Wei, A. Zunger, Appl. Phys. Lett. 64 (13) (1994).
InGaP/GaAs heterojunction bipolar transistor optical and electronic band structure characterization M. Murtagh a,*, J.T. Beechinor a, N. Cordero a, P.V. Kelly a, G.M. Crean a, I.L. Farrell b, G.M. O'Connor b, S.W. Bland c a National Microelectronics Research Centre, Lee Maltings, Prospect Row, Cork, Ireland National Centre for Laser Applications, Department of Experimental Physics, University College Galway, Ireland c Epitaxial Products International Ltd., Pascal Close, Cypress Drive, St. Mellons, Cardiff, South Glamorgan CF3 OEG, Wales, UK b
Abstract In this work we investigate the optical and band structure properties of full InGaP/GaAs based heterojunction bipolar transistor (HBT) epitaxial structures grown by metalorganic chemical vapour phase epitaxy (MOVPE). In related work, full HBTs have been fabricated from the two wafers studied, which exhibit high and low common-emitter current gain (hFE) parameters on electrical test. The focus of this study is to investigate and compare the photoluminescence and photore¯ectance spectroscopy response of these known good and bad epitaxial wafers. The results of low temperature (10±300 K) spectral and transient photoluminescence (PL) analysis are presented, revealing evidence of the nature of the InGaP ordering induced non-radiative loss mechanism. The results also demonstrate the modi®cation to the PL lineshape arising from the InGaP/GaAs interfacial conditions. The experimental results are supported by X-ray diffraction data and ®nite-element device simulation, showing the effect of intermixing layers on the interfacial band potentials. The optical modulation technique of photore¯ectance (PR) spectroscopy was employed to investigate the band structure and interfacial electric ®elds, Fs, of the HBT structures. Following the polarisation±[110] and [11Å0]±dependence of the sub-lattice ordering PR response, it was found necessary to include an emitter/ base intermixing layer in order to account for the InGaP/GaAs Fs data. It is concluded that non-optimal MOVPE growth conditions for one of the structures resulted in both sub-lattice ordering and layer intermixing effects, consistent with the low hFE of the HBTs fabricated from this material. q 2000 Elsevier Science S.A. All rights reserved. Keywords: InGaP/GaAs HBT; Ordering; Intermixing; Photoluminescence; Photore¯ectance
1. Introduction The unique heterojunction properties such as large bandgap, favourable band discontinuities and low interface recombination velocity make the InGaP/GaAs system an attractive candidate for a variety of electronic and optical devices such as heterojunciton bipolar transistors (HBTs) [1]. Moreover it is not easily oxidised, has a high etch selectivity with GaAs when compared to AlGaAs and unlike the latter, no deep level traps, e.g. DX centres have been thus-far reported. Importantly however, is the occurrence of spontaneous `CuPt' type long range ordering in the ternary alloy i.e. tendency of the InGaP cations to order on the group III sub-lattice, producing alternate In and Ga rich (11Å1) and (1Å11) planes [2], with consequent zone folding (L point and zone centre), resulting in a reduced interband
* Corresponding author. E-mail address: [email protected] (M. Murtagh)
transition energy within the emitter layers, as well as polarised (i.e. directional dependent) optical properties [2]. During the growth process itself, attainment of perfectly abrupt InGaP/GaAs interfaces is dif®cult, leading to interfacial mixing as a result of growth precursor `memory' effects, as well as other growth parameter related lattice matching issues including temperature and alloy composition. Both InGaP ordering and InGaP/GaAs layer interfacial mixing may lead to poor HBT devices. Ordering may reduce the carrier injection while interfacial mixing may lead to additional interfacial carrier recombination mechanisms, i.e. where the p±n and metallurgical junctions are not coincident, hence resulting in reduced HBT current gain [3]. Note that under most operating conditions HBT degradation is predominantly current driven [4]. Note also that interfacial mechanical stress arising from the presence of mixing layers, has important implications for electrical property degradation of the HBT device, proving to be a catalyst for defect creation and migration [4].
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0090 2-5
M. Murtagh et al. / Thin Solid Films 364 (2000) 58±63
In this work, we report the results of both a photoluminescence (PL) and a photore¯ectance (PR) optical spectroscopic study of the electronic band structure for two representative HBT structures, showing both good and bad electrical device operation. The spectroscopic results are consistent with the occurrence of both ordering and interfacial mixing for the bad HBT, and demonstrate in particular the importance of inclusion of both effects for a full understanding of the data. 2. Experimental and results The GaAs/InGaP based HBT samples in this study were grown on (100) 28 off-axis toward (110) GaAs substrates using MOVPE, with the following structure: collector contact layer of n 1 GaAs followed by n-GaAs collector, a base layer of p 1 GaAs with a similar thickness n-InGaP emitter, n 1 InGaP emitter contact and ®nally a thick n 1 GaAs cap. Two sample sets were examined, one exhibiting good device performance, the other revealing non-optimal MOVPE growth and poor HBT device characteristics. PL measurements were recorded in a closed cycle cryostat (10 to 200 K), with Ar 1 (488 nm) laser excitation and CCD detection. Transient PL data were recorded using a photon counting PMT detector (5 ns pulse width), with 10 kHz pulsed laser excitation using acoustic-optic modulation, with a matched 5 ns fall time. PR spectroscopy was performed at room temperature (RT) using a twin scanning arrangement [2] over the 1.3± 2.l eV range, with excitation from an Ar 1 (488 nm) laser source. Previous double crystal X-ray diffraction data [5] indicated the presence of InGaP/GaAs interfacial mixing, i.e. between the GaAs cap and InGaP emitter as well as between
59
the emitter and the p 1 GaAs base layer for the poor HBT device. The data in particular revealed an approximate 2 nm In0.38Ga0.62P intermixing layer at the emitter/base interface, correlating with device electrical results showing a variation in the common emitter current gain, hFE, equal to 11 for the poor HBT, compared to a value of 79 for the good HBT device. The results of the poor HBT structure are furthermore consistent with the measured emitter/base current-voltage data, exhibiting large ideality factors, i.e. consistent with non-ideal thermionic emission [6] and possibly arising from a stress induced increase in the carrier recombination [6], but also indicating a possible barrier height voltage dependence, due to the existence of a thin interfacial layer [6]. Fig. 1 presents the PL spectral evolution of both HBT InGaP layer structures as the temperature increases from 10 to 200 K. Two well resolved peaks are observed for both, with a stronger overall luminescent signal recorded for the poor HBT±approximately twice the PL intensity of the nominal HBT at 10 K, and con®rmed by RT measurements. The spectral data are summarised in Table 1, with a full width half maximum (FWHM) of 16 meV recorded for the high energy peak (10 K) for the good HBT, and indicative of good quality material. Arrhenius analysis of the normalised integrated PL intensities yield values for the activation energies of any nonradiative loss mechanism(s), according to [7] R T 1 1A Ce 2EA =kT 1B Ce 2EB =kT 21
1
where EA and EB are the thermal activation energies and AC and BC are effectively the ratio of non-radiative to radiative recombination probabilities for the two loss mechanisms at RT.
Fig. 1. Spectral evolution of PL peaks with temperature for both HBT InGaP emitter layers.
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M. Murtagh et al. / Thin Solid Films 364 (2000) 58±63
Table 1 Summary of LT PL, PR and ®nite-element device model analysis showing the effect of inclusion of intermixing layers upon the electric ®eld and lineshape data a Photoluminescence (10 K) Nominal HBT 1.961 eV peak EA 11.1 meV A C 12.8 EB 62.9 meV B C 499 1.918 eV peak EA 35.5 meV A C 7205 EB 9.53 meV B C 9.41 a
Photore¯ectance (RT) `Poor' HBT
1.89 eV peak 17.7 meV 35.9 60.9 meV 648
Finite-element model
Nominal HBT
HBT `Poor'
Nominal HBT
`Mixing' HBT
E0(InGaP) 1.905eV
1.853 eV
G 8.99 meV
7.87 meV
FC 26 kV/cm
23 kV/cm
FC 26 kV/cm
26 kV/cm
FE 111 kV/cm
108 kV/cm
FE 110 kV/cm
133 kV/cm
The PL (Arrhenius) non-radiative loss mechanisms are summarised along with the corresponding probabilities ( A/BC).
Fig. 2 shows the result of a least-squares regression to the above R(T) equation for the good HBT structure, i.e. for the high energy peak and assuming two activation energies, with results summarised in Table 1. The plot moreover could not be ®t with just one non-radiative mechanism. From Table 1 the dominant activation energy for this peak is 63 meV while for the second lower energy 1.918 eV PL peak, a dominant non-radiative loss mechanism with an energy of around 35.5 meV was observed. The broader low energy peak is rapidly quenched by temperature, moving from 1.918 to 1.896 eV over the temperature range 10 to 70 K. Its energy shift does not follow that of the high energy peak, thus eliminating the possibility of a phonon replica. The spectral evolution for the non-optimal HBT again
reveals a strong low energy component, Fig. 1b, however as the temperature increased the peaks were poorly distinguished due to the spectral broadening of the low peak. The energies of the peaks were established at 1.868 and 1.89 eV, considerably lower then that of the nominal HBT structure and indicative of atomic ordering [7]. The low energy PL furthermore is quenched by 60 K, while the other peak initially blueshifts up to a temperature of about 70 K whereupon it begins to redshift, a behaviour characteristic of ordered material. That is the `turn-around' temperature depends upon the amount of energy required to thermalize the electrons/holes out of the ordered induced band-edge ¯uctuations, into those band-edge states of the randomly ordered crystalline matrix [8]. As a consequence of the spectral broadening of the low
Fig. 2. Arrhenius thermal behaviour of the normalised PL peak data for both HBT InGaP layers, showing the results of a least squares regression to the R(T) equation.
M. Murtagh et al. / Thin Solid Films 364 (2000) 58±63
energy component, Arrhenius data were determined for this sample only after removing data from 20 to 60 K (inclusive) and by plotting the intensities at 10 K and from 70 to 300 K, i.e. from which good ®ts were obtained. As shown in Table 1, the dominant activation energy for non-radiative loss was 61 meV. The higher activation energies of around 60 meV extracted from the high PL transitions of both HBTs have been suggested to be indicative of a deep level trap, arising from possible deep acceptor, neutral impurity or even a vacancy related defect [7]. The most likely cause for the lower activation energies varying from 9.5 to 17.7 meV, has also been speculated to be characteristic of trapped excitons or carriers thermalising from localised regions of ordered induced band-edge ¯uctuations, followed by nonradiative recombination [7]. Interestingly for the good HBT however, when combining both loss energies for the low 1.918 eV peak, the resulting energy (35.5 1 9.5) 45 meV, closely resembles the difference in energy separation of the two PL peaks in question, 43 meV. This is therefore suggestive of the nature of the ordering induced loss mechanism, namely that energy required for the trapped excitons or carriers to thermalise and diffuse to the vicinity of the disordered higher energy band-edge minima, 1.961 eV. The relative activation intensity ratios should be noted however for the two mechanisms, revealing the relative dominance of EA ( AC). However it should also be noted that the ratios AC and BC are not uniquely determined, with the radiative transition probability in general expected to vary as a function of temperature [7]. As shown in Fig. 3, time resolved PL measurements with lifetimes de®ned as the decay interval to 1/e of peak value, were investigated for both HBT InGaP layers. The lifetime
61
of the high energy peak for the good HBT was measured to be 26 ^ 3 ns at an excitation power of 5 kW/m 2. This short decay is consistent with excitonic recombination, as well as with both the observed laser excitation and power spectrum evolution trends. The longer lifetime of 100 ^ 10 ns (7.9 kW/m 2) recorded for the low energy peak is also consistent with some level of spatial carrier separation arising from possible isolation of impurity centres, as indeed indicated by laser excitation experiments. Longer lifetimes of 34 ^ 3 ns at both 7.9 kW/m 2 and 260 W/m 2 excitation levels were recorded for the poor HBT InGaP layers, Fig. 3, again con®rming the ordered nature of the alloy layers. Note that low incident excitation powers (260 W/m 2) were necessary in order to discern the longer lifetimes expected from the ordered sample, i.e. as the ®nite number of states in the ordered domains saturate ± before the faster component due to recombination in the randomly ordered matrix begins to dominate. It is important ®nally, to note the higher luminescent intensity observed for the poor HBT sample, notwithstanding previous studies detailing reduced PL intensities for ordered InGaP [7,8]. The PL response, i.e. both LT and RT band edge data moreover ± assuming no signi®cant dopant variation between samples, suggest some modi®cation from the InGaP/GaAs interfacial conditions. That is consistent with the previously observed intermixing n 1 GaAs/emitter/p 1 GaAs layers, resulting in a possible reduction in the number of interfacial non-radiative recombination states and hence increased PL intensity. Other ordering induced Fermi level pinning effects however may also account for the PL intensity variation, as will be discussed later during the photore¯ectance data. The experimental results are correlated with ®nite element device modelling, i.e. self-consistent Poisson and
Fig. 3. Transient PL response for both HBT emitter layers - the laser intensities are also shown.
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M. Murtagh et al. / Thin Solid Films 364 (2000) 58±63
electron-hole transport solver [9] of space distributed values for the HBT structures. The simulations were performed assuming an InGaP/GaAs conduction band offset of 0.20 eV [1]. As detailed in Table 1, the model results, which includes estimates of photovoltage effects upon the electric ®elds [3], show that both InGaP/GaAs interfacial ®elds increase following the inclusion of interfacial, i.e. previous DCXRD intermixing layers, between both the GaAs cap and InGaP emitter as well as between the emitter and p 1 GaAs base. Speci®cally for the poor HBT, an increase of 23 kV/ cm is observed for the electric ®eld in the base/emitter interface region. Fig. 4 presents the recorded photore¯ectance (PR) data for both HBT structures, with band structure responses from both the GaAs (1.42 eV) and InGaP (<1.9 eV), i.e. collector and emitter HBT regions, respectively. The spectra exhibit typical medium to high surface electric ®eld response as indicated by the exponentially broadened band-edge transition, E0, along with above band-gap Franz±Keldysh oscillations (FKO) [3]. From the FKO period, estimates of the electric ®elds may be extracted, i.e. assuming effective GaAs electron and heavy hole masses of 0.067 and 0.34(001) while for InGaP, masses of 0.118 and 0.66(001), respectively, were employed [10]. The results of a Newton±Gauss least squares regression analysis of a medium ®eld functional form (Airy functions) [3] to the InGaP experimental data are also shown (- - -) in Fig. 4, with extracted spectral parameters, i.e. electric ®eld, Fs, E0 and phenomenological lifetime broadening parameters, G all summarised in Table 1. It should be noted that the in¯uence of slowly varying low-®eld PR structure from highly doped layers was not taken into account in the model. Small reductions in the collector/base ®eld between both HBT's may be a result of lower collector doping for the poor performance HBT, however for the InGaP PR structure, i.e. the base/emitter interface, reductions in both the
transition energy and electric ®eld data are observed. The recorded band-gap shrinkage of about 50 meV is consistent with the PL data, viz. revealing self-ordering effects. The degree of self-ordering may be determined by [8]
h2
E0 h0 2 E0 h h1 DEBGR
2
where E0 h0 1:915 eV and DEBGR h1 0:471 eV [8] and revealing a h value of approximately 0.36 for the poor HBT structure. In order to con®rm this ordering induced band-gap shrinkage however, i.e. not variations in alloy mole fraction, InGaP PR spectra were also recorded for incident light polarised along both [110] and [11Å0] directions. As shown in Fig. 4b, the observed red shift of the [110] polarisation can be used to estimate the degree of ordering, h , i.e. with reference to previous data [8] and determined also to be approximately 35%, corresponding to a polarisation shift of E0110 2 E0110 < 8±10 meV. Thus from the ordering induced piezoelectric effect [8], an estimate of the associated ®eld shrinkage is 25±30 kV/cm. It should be noted however that this ®eld reduction assumes ordering of undoped layers and does not account for possible carrier screening effects from the doped InGaP. Hence when accounting for both an intermixing layer increase in the interfacial ®eld (23 kV/cm), as well as an ordering (piezoelectric) ®eld reduction, a value of 103±108 kV/cm is therefore determined for the emitter/base ®eld, in reasonable agreement with the PR experimental data, 108 kV/cm, Table 1. The level of ordering induced valence-band-splitting (VBS) may be determined from a calculation involving the valence band Hamiltonian [11], and determined for the poor HBT to be 14.9 meV, assuming a crystal ®eld splitting of 0.2 eV and a spin orbit splitting of 0.1 eV [11]. Interest-
Fig. 4. (a) PR spectra for both GaAs collector and InGaP emitter regions for both HBT structures. Best ®t analysis (- - -) are also shown for the emitter PR structure. (b) Spectral shift observed for the polarized PR InGaP response from the ordered HBT.
M. Murtagh et al. / Thin Solid Films 364 (2000) 58±63
ingly therefore unlike previously reported, both the VBS and measured polarisation shift of 8±10 meV differ from each other by more than a factor of two [11], consistent with some level of InGaP/GaAs interfacial strain, i.e. some intermixing in¯uence upon the VBS, as measured from the XRD data. As well as the potential carrier screening effects of the piezoelectric ®eld for doped InGaP, it must also be mentioned that the above results do not take into account other possible ordering induced changes, such as the transition from a type I to a type II band alignment for fully ordered-InGaP/GaAs interfaces [8]. The analysis also assumes little or no ordering induced changes to the effective band masses, m *, i.e. as a result of two competing mechanisms, viz. reduced band-gap (decreased m *) and G ±L band states coupling (increased m *) [8]. The in¯uence of ordering upon m * however is assumed to be negligible, especially given the square root dependence with ®eld [3]. Moreover and most signi®cantly, a theoretical treatment has even suggested that the interface between a large domain of ordered InGaP and a cubic material like GaAs will very likely have the Fermi level (EF) pinned at one of the band edges [8], with the possible excess electrons or holes signi®cantly modifying the offset. This could alternatively explain the apparently anomalously reduced electric ®eld for the poor HBT, i.e. assuming EF pinning near the conduction band at the emitter/base interface, hence leading to an increase in the emitter/base interfacial ®eld. This would also have implications for the previous PL intensity data, that is with conduction band-edge EF pinning, populating possible interfacial non-radiative states, thus leading to a decrease in the ratio of non-radiative to radiative recombination, and hence increased PL intensity, R. 3. Conclusions The photoluminescence (PL) and photore¯ectance (PR) spectral responses of InGaP/GaAs HBT structures of known good and bad device performance (current gain hFE) were compared. The Arrhenius PL study reveals evidence for the
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nature of the InGaP ordering induced non-radiative loss mechanism, while the PR ®eld data was interpreted by including InGaP/GaAs layer intermixing. The results demonstrate in particular the effect of sub-lattice ordering as well as interfacial mixing layers upon both the PL and PR spectral lineshape responses. Further issues relating to the band structural electronic response, including ordering induced Fermi level pinning, were also outlined. It is concluded that non-optimal MOVPE growth conditions for one of the structures resulted in both sub-lattice ordering and layer intermixing effects, consistent with the low hFE of the HBTs fabricated from this material. Further work will be required to elucidate the exact mechanisms in order to correlate the results with those of the electrical device data, as well as to investigate the relative dominance of ordering or intermixing upon HBT device performance. Acknowledgements Brite-EuRam project: HEROS, contract number BRPRCT98-0789. References [1] H. Kroemer, J. Vac. Sci. Technol. B. 1 (1983) 126. [2] E. Gregor, K.H. Gulden, M. Moser, G. Schmiedal, P. Kiesel, G.H. Dohler, Appl. Phys. Lett. 70 (11) (1997) 1459. [3] F.H. Pollak, W. Krystek, M. Leibovitch, H. Qiang, Am. Inst. Phys. (1996) 669. [4] T. Henderson, W.L. Chen, M. Sanna, Future Fab, Int. (1997) 229. [5] E. Richter, F. Brunner, S. Gramlich, S. Hahle, M. Mai, U. Ziemer, M. Weyers, Invited Paper, Symp. 5, EXMATEC `98 4th Int. Workshop on Cont. of Comp. Semi. Mater. Technol., 21st±24th June, Cardiff, Wales. [6] K. McCarthy, C. Lyden, W.M. Kelly, Solid-State Electron. 34 (2) (1991) 220. [7] J.D. Lambkin, L. Considine, S. Walsh, G.M. O'Connor, C.J. McDonagh, T.J. Glynn, et al., Appl. Phys. Lett. 65 (1) (1994). [8] S. Froyen, A. Zunger, A. Mascarenhas, Appl. Phys. Lett. 68 (1996) 2852. [9] Silvaco Data Systems: Santa Clara, CA. [10] Y. Zhang, A. Mascarenhas, Phys. Rev. B 51 (1995) 13162. [11] S.H. Wei, A. Zunger, Appl. Phys. Lett. 64 (13) (1994).