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2.3.1

Junction formation and characterisation.

The amphoteric doping nature of silicon on various surfaces has be exploited in the fabrication of lateral p-n junction structures by single-step growth. Between surface of differing conductivities grown under the same growth conditions, a lateral p-n junction is created. By the use of suitably patterned metallisation patterns, lateral junctions can be electrically isolated and characterised by Current-Voltage (l-V) and Capacitance-Voltage (C-V) profiling.

The first study of lateral p-n junctions formed by silicon doped GaAs overgrowth on a patterned (100) substrate was conducted by Miller [97]. Chemical etching was use to create a ridged structure on the GaAs (100) substrate, exposing (111)A planes between the flat (100) surfaces. Silicon

doped material grown over these surfaces was doped p- and n-type respectively, thus creating p-n junctions at the surface boundaries. A (411)A surface developed at the upper facet-flat boundary during growth and it was demonstrated that the upper junction had relocated onto the facet and was situated between the (111)A and (411 )A surfaces. The growth behaviour and junction position was not commented upon. Our studies also found this junction relocation, which is described in Chapter 5. The lower junction was positioned between the (111)A sidewall and (100) surface, as would be expected from the 'as-etched' substrate morphology. Breakdown characteristics of the n-p-n 'double-diode' structure were reported and different current behaviour was seen, dependant on the voltage direction.

Cross-sectional EBIC measurements reported in a paper by Takamori and Kamijoh show that the upper junction on a (100)-(311)A patterned substrate had also moved onto the facet [71]. It was now situated between the p-type (311)A sidewall and a new n-type (411)A facet, which had nucleated from the (311)A- (100) boundary. The lower p-n junction was at the intersection of the (100) and (111)A surfaces, the latter developing during growth at the lower facet-flat boundary [71]. Electrical breakdown across the n-p-n 'double-diode' structure exhibited dissimilar characteristics, dependant on the bias direction. This behaviour is very similar to that seen but not commented upon by Miller [97]. Differing junction doping profiles were proposed to explain the observed breakdown behaviour; the (411)A surface being heavily compensated and thereby forming a graded structure compared to the abrupt lower junction formed between uncompensated p-(111)A and n-(IOO) surfaces [71].

A series of papers have been published on the electrical characteristics of lateral p-n junctions formed on patterned (111)A substrates [98-103]. Utilising the three-fold symmetry of the (111)A surface, the authors etched triangular structures, both recessed and protrudent, possessing a (111)A top surface and (311)A sidewalls [98]. Si-doped GaAs was grown over these mesas, doping the sidewalls n-type and the flats p-type. Cathodoluminscence measurements on the junctions demonstrated that the conductivity varied more abruptly at the upper junction than the lower one and also revealed that increasing the As4:Ga

ascribed this behaviour to Ga adatom migration effects during growth, although this was not discussed in detail [98]. Gurrent-voltage measurements across the n-p-n ‘double-diode’ structure showed that the recessed structure possessed a higher leakage current than the protrudent one. This was attributed to leakage paths through the rounded corners of the recessed device.

In a following paper, a detailed electrical characterisation of the (111)A- (311 )A upper junction was presented [99]. Substrate growth temperature (Tg)

was found to have a dramatic effect on both junction profile and electrical performance. Under reverse bias, the breakdown voltage and leakage current increased with higher values of Tg, pointing to wider depletion widths at higher growth temperatures. A similar effect was witnessed in the forward direction under low biases, with an unidentified current component evident in the l-V characteristics [99]. No explanation over the origin of this current was proposed and its magnitude was seen to increase with growth temperature. Capacitance- voltage profiling revealed that the lateral junction had a graded doping profile, with the profile tending towards abrupt as the growth temperature was reduced

[99]. A reduction in the local As4:Ga ratio by migration of Ga atoms from the

(111)A to (311)A surface was proposed to explain the growth temperature dependence. Excess As present on the (111)A surface and excess Ga on the (311)A favouring compensation of the doped material in the junction region [99].

As the excess Ga population decreased with distance away from the surface boundary, the carrier density would rise to its uncompensated value a certain distance from the intersection. A graded carrier distribution was therefore created, whose extent was dependant on the Ga diffusion length, which exhibited the correct temperature dependence [99].

Light emission was reported from the bulk lateral (111)A junctions [100-101]. Increased output power levels at high doping concentrations were recorded for the lower junction compared to the upper junction. This was attributed to increased compensation in the upper junction region, leading to non-radiative recombination and reduction of the optical output [100]. In a related study, the optical and electrical properties of the lateral (111)A junctions were compared to conventional double-dopant p-n junctions [101]. The lateral junctions displayed superior characteristics, which was attributed to a more abrupt junction profile

due to the low diffusivity of silicon.

2.3.2

Hetrostructure and Delta-doped devices.

Delta-doped and Multi-Quantum Well (MOW) devices were fabricated on patterned (111 )A substrates [99,102-103]. Direct interband tunnelling and Negative Differential Resistance were observed in the forward characteristics of the delta-doped junction. Within the heterostructure device, it was proposed that the junction was formed between quantised electron states in the n-type material and quantised hole states in the p-type material, as depicted in Figure 2.9. Carriers were constrained in the vertical direction by the wider bandgap barrier material and horizontally by the p-n junction [102]. The MOW device operated as an efficient Light Emitting Diode, with emission evident from the (111)A surface due to recombination between injected electrons and holes resident in the p- type subband [102-103]. No emission was observed from the (311)A facet and it was proposed that hole injection was blocked by a valence band discontinuity due to the difference in quantum well thickness at the (111)A/(311)A interface [103]. However, direct tunnelling was not seen in the MWQ l-V characteristics. This casts doubt over the quantised nature of the p-n junction, as one would expect direct tunnelling to occur between the quantised states in conduction and valence band [102]. ( 1 1 1 ) A p Barrier c) Barrier a) A IG aAs AIG aAs a) Cross-section of LSJ device b) Energy-band diagram

- lateral confinem ent c) Potential energy

- vertical confinem ent

P a tte rn e d G a A s S u b s tra te

ho es

electrons

Figure 2.9. Quantum Well lateral p-n junction.

Lateral tunnelling transistors were fabricated by the application of a Ti/Au 39

gate above the heavily-doped lateral p-n junction [104-105]. For devices formed on (111)A patterned substrates, only those possessing (511)A sidewalls exhibited NDR and it was not seen on (311)A or (211)A facets. The authors proposed that the formation of an uncompensated n-type (411)A surface at the (111)A-(511)A boundary generated the requisite abrupt junction for direct tunnelling [104]. This growth behaviour was not seen on the other sidewalls, where large-scale compensation in the junction region reduced the tunnelling probability. Application of bias to the gate modulated the peak current [104]. Similar results were obtained for device structures fabricated on p-type (311)A flat surfaces and n-type (100) facets [104] and n-type (411)A flat surfaces with p- type (311 )A sidewalls [105].

A complementary PET device was reported by Li and Bhattacharya [106]. They formed the p-channel PET on an etched (311)A facet and the n-type device on the adjoining (100) plane. Low device current in the p-channel device was attributed to partial compensation on the (311)A surface, which reduced the hole concentration and therefore limited the current carrying capacity of the device [106].

Modulation of the confinement energy in quantum wires by lateral two- dimensional p-n junctions has been reported [110-111]. The wires were fabricated on (311)A and (100) GaAs substrates and comprised, either a 2- Dimensional hole gas (2DHG) bounded by two 2D electron gases [110] , or visa versa [111]. Silicon was used to dope the n- and p-type regions in a single-step growth. Reverse biasing the 2DHG gates caused the channel resistance to rise, thus demonstrating that the conducting width of the facet had been reduced by the depleting action of the lateral junctions.

2.3.3

Laser structures.

Several types of laser structure incorporating the amphoteric doping properties of silicon in GaAs have been reported [95,107-109]. The different conductivity types on interconnected surfaces have been used to define a current injection path, with the lateral junctions acting as the current-constraining mechanism. A diagram of one such device is shown in Figure 2.10.

n-type metallisation AIGaAs:Si P ' electron injeciion ' P ^

i n i

active layer; AIGaAs undoped, GaAs Quantum Well, AIGaAs undoped

Figure 2.10. Quantum Well laser with current-constraining lateral p-n junctions.

This configuration limits the spreading resistance and reduces the active region to very small dimensions, facilitating single mode lasing operation [95]. Miller and Asbeck used the p-type facet as the hole injection path in their structure [107]. However, lasing action was not seen in this device or the Transverse-Stripe-Junction laser reported by Meier et al [107-108]. Low optical efficiency due to non-radative recombination in the junction region and poor morphology of the grown p-type layer, reducing the optical feedback were proposed in explanation of this effect. A HEMT-type laser was fabricated by Ebner et al [109]. The junction was formed between a 2-D hole gas on the p- type (311)A facet and a 2-D electron gas on the (100) flat surface.

2.4. Summary

• Silicon doping of different surface orientations of GaAs has been reviewed. • Surface configuration and growth conditions has been shown to influence the

conductivity type of the Si-doped GaAs.

• The normal conductivity type of MBE grown silicon-doped GaAs is n-type. • P-type Si-doped GaAs can be grown on (N11)A i.e.N=1-4 and (110) surfaces

under the correct growth conditions.

• Carrier density saturates with increasing silicon concentration and defect centres which cause this behaviour were reviewed.

• The different growth behaviour of GaAs, AlAs and AIGaAs over non-planar surfaces was discussed and facet generation due to preferential growth of certain crystal surfaces described. Structures which exhibit quantum effects have been fabricated using this effect.

• The formation of lateral p-n junctions can be achieved by growing silicon- doped GaAs over the appropriate combination of interconnected surfaces under the correct growth conditions.

• Published literature on the production of lateral p-n junctions on non-planar substrates was reviewed and results relevant to our studies discussed.