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A UNIVERSAL DAMAGE INDUCED TECHNIQUE FOR QUANTUM WELL INTERMIXING

We report a novel technique for quantum well intermixing which is simple, reliable and low cost, and appears universally applicable to a wide range of material systems. The technique involves the deposition of a thin layer of sputtered SiO2 and a subsequent high temperature anneal. The deposition process appears to generate point defects at the sample surface, leading to an enhanced intermixing rate and a commensurate reduction in the required anneal temperature. Using appropriate masking it is possible to completely suppress the intermixing process, enabling large differential band gap shifts (over 100 meV) to be obtained across a single wafer.

Short‐circuit current and energy efficiency enhancement in a low‐dimensional structure photovoltaic device

We have studied the forward bias behavior of AlGaAs/GaAs p‐i‐n multiquantum well (MQW) photodiodes. In samples with low background impurity levels in the intrinsic region the high quantum efficiency observed in reverse bias is maintained into forward bias even for carriers photoexcited in the wells. We compare our MQW devices with structures which are identical apart from having AlGaAs intrinsic regions without quantum wells. The short‐circuit currents in the MQW structures are much higher than in the control samples though the open‐circuit voltages are somewhat smaller. In one case the energy conversion efficiency of the MQW device in white light is 110% higher than the control. We discuss the implications of our results for the development of low‐dimensional structure solar cells.

Steady-state carrier escape from single quantum wells

The authors have studied the variation in DC photocurrent with bias and temperature from GaAs-Al/sub x/Ga/sub 1-x/As single quantum wells embedded in p-i-n diodes. They found that the observed temperature response shows Arrhenius behaviour with a field-dependent activation energy close to the hole well depth. This can be accounted for using a model based on the competition between photocarrier escape and recombination. Using reasonable values for the diodes built-in voltage and the quantum-well recombination lifetime, good quantitative agreement between theory and experiment is achieved if it is assumed that the recombination rate is governed by the fastest escaping carriers, which are light holes in the present devices. >

Monolithic integration in InGaAs-InGaAsP multiple-quantum-well structures using laser intermixing

The bandgap of InGaAs-InGaAsP multiple-quantum-well (MQW) material can be accurately tuned by photoabsorption-induced disordering (PAID), using a Nd:YAG laser, to allow lasers, modulators, and passive waveguides to be fabricated from a standard MQW structure. The process relies on optical absorption in the active region of the MQW to produce sufficient heat to cause interdiffusion between the wells and barriers. Bandgap shifts larger than 100 meV are obtainable using laser power densities of around 5 W/spl middot/mm/sup -2/ and periods of illumination of a few minutes to tens of minutes. This process provides an effective way of altering the emission wavelengths of lasers fabricated from a single epitaxial wafer. Blue shifts of up to 160 nm in the lasing spectra of both broad-area and ridge waveguide lasers are reported. The bandgap-tuned lasers are assessed in terms of threshold current density, internal quantum efficiency, and internal losses. The ON/OFF ratios of bandgap-tuned electroabsorption modulators were tested over a range of wavelengths, with modulation depths of 20 dB obtained from material which has been bandgap-shifted by 120 nm, while samples shifted by 80 nm gave modulation depths as high as 27 dB. Single-mode waveguide losses are as low as 5 dB/spl middot/cm/sup -1/ at 1550 mm. Selective-area disordering has been used in the fabrication of extended cavity lasers. The retention of good electrical and optical properties in intermixed material demonstrates that PAID is a promising technique for the integration of devices to produce photonic integrated circuits. A quantum-well intermixing technique using a pulsed laser is also demonstrated.

HIGH QUALITY WAVELENGTH TUNED MULTIQUANTUM WELL GAINAS/GAINASP LASERS FABRICATED USING PHOTOABSORPTION INDUCED DISORDERING

Broad area oxide strip lasers have been fabricated from GaInAs/GaInAsP multiquantum well laser material which has undergone various degrees of intermixing by photoabsorption induced disordering. This process provides an effective way of altering the emission wavelength of lasers fabricated from a single epitaxial wafer, and we have demonstrated blue shifts of up to 160 nm in lasing spectra. The band gap tuned lasers are also assessed in terms of threshold current density, internal quantum efficiency, and internal loss and it is shown that good device performance is maintained.

Avalanche multiplication in GaInP/GaAs single heterojunction bipolar transistors

The electron multiplication factors in GaInP/GaAs single heterojunction bipolar transistors (HBTs) have been measured as a function of base-collector bias for a range of GaAs collector doping densities. In the lowest doped (5/spl times/10/sup 14/ cm/sup -3/) thick collector the multiplication is determined by the local electric field. As the collector doping increases, the measured multiplication is found to be significantly reduced at low values of multiplication from that predicted by the electric field profile. However, good agreement is always found at high multiplication, close to breakdown. This reduction in multiplication at low electric fields is attributed to the dead space, the minimum distance over which carriers must travel before gaining the ionization threshold energy. A simple correction for the dead space is proposed, allowing the multiplication to be accurately predicted even in heavily doped structures.

MOVPE growth of AlGaAs using trimethylamine alane

Abstract Single layers of AlGaAs and quantum wells of GaAs/AlGaAs have been grown from the adduct trimethylamine alane (NMe 3 ·AlH 3 ), trimethylgallium (TMG) and arsine (AsH 3 ) using atmospheric pressure MOVPE. The growth is characterised by rapid darkening of the reactor due to a homogeneous side reaction. Low temperature photoluminescence data from the alloy Al 0.33 Ga 0.67 As shows that there is no reduction in the residual carbon contamination compared to a similar layer deposited from trimethylaluminium, TMG and AsH 3 . The trend in electrical properties indicates an increasing acceptor concentration arising from the high purity TMG source. We have proposed that methyl groups from TMG are the precursors to the observed carbon contamination in NMe 3 ·AlH 3 grown AlGaAs.

ANGULAR SPECTRUM OF VISIBLE RESONANT CAVITY LIGHT-EMITTING DIODES

We present spectrally resolved angular radiation patterns from visible resonant cavity light-emitting diodes. Recording the pattern for a fixed wavelength clearly demonstrates how the overall angular emission is strongly influenced by the relative spectral alignment of the cavity resonance and the underlying quantum well emission, i.e., the detuning. Combined with measurements of total optical power, taken over a range of solid angles, the results highlight the importance of accounting for the collection optics of a proposed application when an optimum design is considered.

Steady state photocurrent and photoluminescence from single quantum wells as a function of temperature and bias

We have studied the variation with applied bias and temperature of steady state photoluminescence (DCPL) and photoconductivity (DCPC) from a series of GaAs/AlGaAs single quantum well, p-i-n structures with different well widths. We present the DCPC and DCPL results, which when combined, allow us to assess how significant nonradiative recombination is in the samples and hence the quality of the material. We discuss the qualitative features in the light of a new theoretical approach presented here for the first time. This includes contributions from escape (of both electrons and holes) and makes it possible to extract from the experimental data two parameters, each reflecting the competition between escape and one of the recombination processes (radiative or nonradiative) in the absence of the other. We further comment qualitatively on the bias and temperature dependence of these different processes.

Space charge effects in carrier escape from single quantum well structures

Recently published data on the variation with applied bias and temperature of steady-state photoluminescence and photoconductivity from a series of GaAs/AlGaAs single quantum well p-i-n structures are subjected to detailed theoretical analysis, using phenomenological variables introduced in connection with these results. The data are interpreted as revealing the presence in the well of a space charge, which causes band bending and hence indirectly modifies carrier escape lifetimes. It is shown that the thermionic escape of holes can affect the electron tunneling escape lifetime so that the latter displays a thermal activation energy which is quantitatively similar to the hole well depth.