GaAs based heterojunction bipolar transistor characterisation using non-contact optical spectroscopy

Materials Science and Engineering B66 (1999) 185 – 188 www.elsevier.com/locate/mseb

InGaP/GaAs based heterojunction bipolar transistor characterisation using non-contact optical spectroscopy M. Murtagh a,*, J.T. Beechinor a, N. Cordero a, P.V. Kelly a, G.M. Crean a, S.W. Bland b b

a National Microelectronics Research Centre, Lee Maltings, Prospect Row, Cork, Ireland Epitaxial Products International Ltd., Pascal Close, Cypress Dri6e, St. Mellons, Cardiff, South Glamorgan CF3 OEG, Wales, UK

Abstract In this work we have characterised InGaP/GaAs based heterojunction bipolar transistor (HBT) structures, fabricated by metalorganic chemical vapour phase epitaxy (MOVPE), using non-contact electro-optic spectroscopic techniques, photoreflectance (PR), photoluminescence (PL) and ellipsometry. PR analysis details both band structure and interfacial electric field data for both the GaAs collector and InGaP emitter regions. Including sub-lattice ordering for the InGaP alloy, the PR analysis also indicates the presence of an interfacial or intermixing layer between the emitter and GaAs base as a result of non-optimal MOVPE growth. The results are compared with room temperature PL spectra, demonstrating in particular the modification to the PL lineshape arising from the InGaP/GaAs interfacial conditions. The experimental data are also supported with finite-element device simulation, showing the effect of mixing layers on the interfacial band potentials. To interpret the experimental HBT ellipsometric response it was found necessary to include InGaP/GaAs layer intermixing within the optical model, consistent with the previous data. The implications of these results for HBT device performance are also discussed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: InGaP/GaAs based heterojunction bipolar transistor; Metalorganic chemical vapour phase epitaxy; Photoreflectance; Photoluminescence

1. Introduction Heterojunction bipolar transistors (HBT) are well established devices for a wide range of microwave and power applications as well as for various digital and non-linear applications such as low phase noise oscillators. The conventional GaAlAs/GaAs HBT has been used most widely to date. However HBT’s based upon the InGaP/GaAs material system [1] demonstrate several fundamental advantages over that of AlGaAs/ GaAs, namely: improved carrier injection into the base because of higher InGaP band-gap, a more favourable conduction band alignment at the heterojunction (0 0.2 eV), the absence of DX centres as well as lower surface recombination velocity. Furthermore this material system has important processing attributes such as lower reactivity of InGaP with oxygen (with respect to AlGaAs) and better etch selectivity between InGaP and GaAs. * Corresponding author.

The current practise of obtaining dopant concentrations and device characteristics, such as current gain and breakdown voltage from the fabrication of test structures is costly and time consuming. These parameters are necessary in order to assess the quality and suitability of the material for device processing. The application of non-destructive, non-contact, non-intrusive characterisation or monitoring tools could have a significant impact upon the cost and yield of devices. Such diagnostic tools should be capable of dealing with the complex nature of semiconductor growth including multi-layer, binary and ternary alloys, compositional grading and other features representative of advanced HBT technology. Optical probes appear ideally suited to the task, proving to be sensitive to material properties [2] as well as various processing steps which are related to actual device parameters and performance [3]. In this work we report the application of optical spectroscopic techniques including photoreflectance

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(PR), photoluminescence (PL) and ellipsometry for qualification of InGaP/GaAs multi-layer HBT material. These techniques reveal important information regarding the quality of the different InGaP and GaAs layers for the emitter, base and collector regions, including atomic ordering effects. As well as characterisation of both the collector and emitter dopant concentrations, PR analysis of the InGaP band structure in particular reveals clear evidence of interfacial mixing at the InGaP/GaAs heterojunction. Evidence of the existence of this intermixing layer is further supported by both the PL and ellipsometric data. Electrical studies of identical samples reveal how both this layer and the atomic ordering prove to have adverse consequences for the current gain characteristics of the resultant HBT device, consistent with measurements of the DC characteristics, i.e. common emitter current gains [4].

2. Experimental and results The GaAs/InGaP based HBT samples in this study were grown on (100) 2° off-axis toward (110) GaAs substrates using metal-organic vapour phase epitaxy (MOVPE) with the following structure: collector contact layer of n+ GaAs followed by n− GaAs collector, a base layer of p+ GaAs with a similar thickness n− InGaP emitter followed by a n+ InGaP emitter contact and finally a thick n+ GaAs cap. In this work two sample sets were examined, one exhibiting good device performance, the other revealing non-optimal MOVPE growth with poor HBT device characteristics. Photoreflectance (PR) spectroscopy was performed at room temperature (RT) using a twin scanning arrangement [2] over the 1.3–2.1 eV range, with excitation from an Ar+ (488nm) source. Variable angle spectroscopic ellipsometry (VASE) was performed from 1.5 to 4.8 eV at incidence angles of 65, 70 and 75°. Photoluminescence

(PL) measurements were recorded at room temperature using the 514 nm line of an Ar+ laser. Fig. 1 presents the ellipsometric response for both the nominal HBT layer structure as well as from the HBT exhibiting poor device performance. The experimentally recorded C and D responses [2] are shown for each sample along with the results of a Marquardt–Levenburg non-linear least squares regression to the optical model employed for interpretation of the experimental response, also in Fig. 1. Extracted layer thicknesses are in good agreement with nominal values, however in order to achieve reasonable fits for the non-optimal HBT, it was found necessary to include interfacial mixing or intermixing layers at both InGaP/GaAs interfaces. These consist of an effective-medium-approximation (EMA–Bruggeman) of the optical properties of GaP and InP at the base/emitter interface as well as an EMA of GaAs and InP, i.e. InGaAsP layer at the emitter/GaAs cap interface. It is important to note that both intermixing layers exhibit significant parameter correlation effects, i.e. it is not possible to uniquely determine the exact composition and layer thickness, however the composition of the emitter/base interfacial mixing layer could be determined to consist of a higher GaP/InP concentration ratio viz. In0.2GaP0.8. The results of RT PL analysis of both HBT structures are summarised in Table 1, revealing both a significant shrinkage of the InGaP band-edge (E0), by about 50 meV, as well as an increase in the PL integrated intensity and lineshape broadening (G), for the HBT structure exhibiting poor device performance. Both E0 and G data are indicative of atomic ordering, i.e. tendency of the InGaP cations to order on the group III sub-lattice, producing alternate In and Ga rich (11( 1) and (1( 11) planes [5] with consequent zone folding (L point and zone centre), resulting in a reduced interband transition energy within the emitter layer. Notwithstanding the ordering, the integrated PL inten-

Fig. 1. Ellipsometric response— C, D at 70°—for both the nominal HBT structure (a), as well as from the HBT displaying poor device performance, (b). Also shown are best-fit optical models (---), incorporating additional InGaAsP and In0.2GaP0.8 intermixing layers for sample (b), extracted as a result of simultaneous least squares fitting of these models to the experimental data at three angles of incidence.

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Table 1 Summary of PR, PL and finite-element device model analysis showing the effect of inclusion of intermixing layers upon the electric field and lineshape dataa Photoreflectance

Photoluminescence

Finite-element model

Nominal HBT

‘Poor’ HBT

Nominal HBT

‘Poor’ HBT

Nominal HBT

‘Mixing’ HBT

E =1.905 eV G= 8.99 meV – FC = 26 kV/cm FE = 111 kV/cm

1.853 eV 7.87 meV – 23 kV/cm 108 kV/cm

E0 = 1.89 eV G= 30 meV Integrated Int. (×105) =0.57 – –

1.84 eV 35 meV 1.0 – –

– – – FC =26 kV/cm FE =110 kV/cm

– – – 26 kV/cm 133 kV/cm

a

Note that the device model includes photovoltage effects to account for the PR pump and probe light sources. Fields are estimated to within9 3 kV/cm.

sity data suggest possible confinement of carriers in the InGaP (E0 = 1.85 eV) emitter, possibly resulting from the In0.2GaP0.8 (E0 ]2.05 eV) intermixing layer as suggested from the VASE results. The experimental results are correlated with finite element device modelling, i.e. self-consistent Poisson and electron-hole transport solver [6] of space distributed values for the HBT structures —assuming an InGaP/GaAs conduction band offset of 0.20 eV. As detailed in Table 1, the model results, which includes estimates of photovoltage effects upon the electric fields [3], show that both InGaP/GaAs interfacial fields increase following the inclusion of the intermixing layers. Specifically, an increase of 23 kV/cm is observed for the base/emitter interface region, consistent with the integrated PL intensity data above. Fig. 2 presents the recorded photoreflectance (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 field response as indicated by the exponentially broadened band-edge transition, E0, along with above band-gap Franz-Keldysh oscillations [2,3], from which electric fields may be extracted, 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. The results of a Newton–Gauss least squares regression analysis of a medium field functional form (Airy functions) [2] to the InGaP experimental data are also shown (---) in Fig. 2, with extracted spectral parameters, i.e. electric field, Fs, E0 and phenomenological lifetime broadening parameters, G, summarised in Table 1. It should be noted that the influence of slowly varying low-field PR structure from highly doped layers [2] was not taken into account in the model. Small reductions in the collector/base field 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 field data are observed. The recorded band-gap shrinkage of about 50 meV is consistent with the PL data revealing self-ordering effects, note however the slight underestimates in the extracted RT PL band-edge data with respect to the PR values, consistent with the more accurate RT derivative nature afforded by the PR

Fig. 2. (a) PR spectra for both GaAs collector and InGaP emitter regions for both HBT structures. Best fit analysis (---) are also shown for the emitter PR structure. (b) Spectral shift observed for the polarised PR InGaP response from the ordered HBT.

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technique [2,3]. In order to confirm this ordering induced band-gap shrinkage, PR spectra were also recorded for incident light polarised along both [110] and [11( 0] directions. As shown in Fig. 2(b) the red shift of the [110] polarisation can be used to estimate the degree of ordering, h, given by [7]: = 0) (h = 1) h 2 = E (h −E (h) 0 0 /DE BGR =1 where E h0 = 0 =1.915 eV and DE hBGR =0.471 eV. Our data ( 0] reveals a h value of approximately 0.3 —E [11 −E [110] : 0 0 8− 10 meV—and thus from the piezoelectric effect [7] an estimate of the associated field shrinkage is about 25 –30 kV/cm [7]. It should be noted however that this field reduction assumes ordering of undoped layers and does not account for carrier screening effects from doped InGaP. Thus, when accounting for both an intermixing layer increase in the interfacial field as well as an ordering (piezoelectric induced) field reduction, a value of 103– 108 kV/cm is determined for the emitter/base field, in reasonable agreement with the PR experimental data, 108 kV/cm, Table 1. The effect of both ordering as well as interfacial mixing between emitter and base will be to reduce carrier injection as well as lead to possible increased interfacial carrier recombination mechanisms, that is where the p – n and metallurgical junctions are not coincident, resulting in reduced HBT current gain [3]. Note that under most operating conditions HBT degradation is predominantly current driven [8]. With regard to the mixing layers, interfacial mechanical stress also has important implications for electrical property degrada-

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tion of the HBT device, proving to be a catalyst for defect reation and migration [8].

3. Conclusion In summary, RT ellipsometric, photoluminescence and photoreflectance studies of HBT structures reveal both atomic ordering in the InGaP emitter as well as nonabrupt InGaP/GaAs interfacial regions-as a result of non-optimal MOVPE growth-consistent with poor HBT device operation. This work demonstrates the application of non-destructive, rapid and non-contact (optical based techniques) for evaluation and control of compound semiconductor materials for HBT technology.

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