Geoffrey Duggan

A new approach to high‐efficiency multi‐band‐gap solar cells

The advantages of using multi‐quantum‐well or superlattice systems as the absorbers in concentrator solar cells are discussed. By adjusting the quantum‐well width, an effective band‐gap variation that covers the high‐efficiency region of the solar spectrum can be obtained. Higher efficiencies should result from the ability to optimize separately current and voltage generating factors. Suitable structures to ensure good carrier separation and collection and to obtain higher open‐circuit voltages are presented using the (AlGa)As/GaAs/(InGa)As system. Efficiencies above existing single‐band‐gap limits should be achievable, with upper limits in excess of 40%.

A photoluminescence study of beryllium‐doped GaAs grown by molecular beam epitaxy

The 300‐K photoluminescence internal quantum efficiency of beryllium‐doped GaAs grown by molecular beam epitaxy at 580 °C from As4 under arsenic stable conditions is limited by nonradiative recombination with τNR = 3.5 to 5.6×10−10 s. τNR remains in this range while 1.3×1015 <  NA−ND  < 8×1017 cm−3. Layers grown at fixed NA−ND = 3×1016 cm−3 and constant thickness exhibited a 140‐fold increase in internal efficiency as the growth temperature was raised from 480 to 660 °C. The corresponding internal efficiencies were calculated to be 0.054%,  ⩾ 0.69%, and  ⩾ 7.6%, respectively. The 6.5‐K photoluminescence spectra of layers grown at 580, 620, and 660 °C showed no significant differences and exhibited none of the new bound exciton lines reported by Kunzel and Ploog to be present in molecular beam epitaxial GaAs grown from As4 below 610 °C.

Experimental confirmation of a sum rule for room‐temperature electroabsorption in GaAs‐AlGaAs multiple quantum well structures

Measurements of the electric field dependence of room‐temperature photocurrent in a GaAs‐A10.4Ga0.6As multiple quantum well p‐i‐n diode have been carried out in the wavelength range 650–920 nm. Calculations of the field dependence of absorption from the photocurrent spectra show that the recently derived sum rule for electroabsorption in quantum wells holds to within 0.3%, for electric fields as high as 2×105 V/cm, provided that full account is taken of changes in absorption across the entire spectral region.

The efficiency of photoluminescence of thin epitaxial semiconductors

General theoretical expressions appropriate to calculating the internal and external quantum efficiency of photoluminescence (PL) are derived for a semiconductor of thickness d and bounded by two surfaces with different surface‐recombination velocities. The external efficiency expected from thin and thick layers is compared and contrasted. Under certain conditions and adopting reasonable assumptions, we indicate how a single measurement of the steady‐state PL can yield information about the minority‐carrier diffusion length.

Optical measurements of electronic bandstructure in AlGaInP alloys grown by gas source molecular beam epitaxy

A series of bulk (AlyGa1−y)0.52In0.48P epilayers, covering the full range of compositions from y=0 to y=1, have been grown lattice matched on GaAs substrates by gas source molecular beam epitaxy. Double crystal x‐ray diffraction, and low temperature photoluminescence (PL) and photoluminescence excitation (PLE) optical spectroscopy have been used to characterize the structures. PL and PLE data indicate the direct‐to‐indirect energy gap crossover composition to be near y∼0.55 for this alloy system. From PLE, the y dependence of the (5 K) lowest energy direct gap, EΓ–Γ, has been found to be 2.014+0.499y+0.16y2 eV. EΓ–Γ (5 K) for the indirect‐gap ternary end‐member Al0.52In0.48P is directly determined to be 2.685 eV.

Some aspects of GaN growth on GaAs(100) substrates using molecular beam epitaxy with an RF activated nitrogen-plasma source

We have investigated how supplying active nitrogen from an RF activated plasma source under various plasma conditions influences certain aspects of the growth of GaN films on GaAs(100) substrates, using molecular beam epitaxy. In the first instance, the quantity of active nitrogen generated by the source was found to have a strong dependence on both the RF power and amount of nitrogen gas supplied to the plasma. In addition, the degree of optical discharge from the plasma was observed to give a semi-quantitative measure of active nitrogen. No observable loss of nitrogen from the sample surface in the temperature range 450 to 680°C was found during GaN growth. Scanning electron microscopy on the cleaved edges of the GaNGaAs(100) samples showed the GaN layer to be polycrystalline with a columnar nature typical of a highly lattice mismatched material system. X-ray diffraction measurements indicated that the GaN layers were entirely wurtzite in structure, with the full width at half maximum of the GaN (0002) reflection in the range 9 to 11.5 arcmin. A broad peak centred at around 3.4 eV was recorded using room temperature photoluminescence measurements on the layers.

Positions of the sub-band minima in GaAs(AlGa)As quantum well heterostructures

Abstract We have combined measured 5K excitonic absorption peaks with calculations of the confined particle states to determine the band offsets in GaAs(AlGa)As quantum well heterostructures. Our observations are best fitted with a conduction band-valence band split of about 75:25. This contrasts with the currently accepted value of 85:15 and the recently proposed value of 60:40.

Uniform and efficient GaAs/AlGaAs quantum dots

Uniform arrays of approximately 57 nm diam free‐standing quantum dots have been produced from GaAs/AlGaAs single quantum well material by electron‐beam lithography and low damage electron cyclotron resonance plasma etching. Low‐temperature (5 K) photoluminescence and photoluminescence excitation spectroscopy were used to characterize the material before and after processing into quantum dots. Clear free‐exciton features of linewidth comparable to that obtained from the unprocessed material have been observed in the excitation spectra of the quantum dots. As expected for this size of dot, no significant shift in the wavelength of the luminescence was observed, however, there is an apparent enhancement of the external luminescence efficiency when the geometric fill factor is taken into account. The results also show that luminescence efficiency measurements seeking to identify and elucidate intrinsic 0D effects (i.e., those due to quantum confinement in the active region) should be performed with photoexcit...

BAND-OFFSET DETERMINATION FOR GAINP-ALGAINP STRUCTURES WITH COMPRESSIVELY STRAINED QUANTUM WELL ACTIVE LAYERS

(AlGaIn)P‐on‐GaAs structures incorporating compressively strained Ga1−xInxP (x≳0.48) quantum well active layers have been studied by low‐temperature photoluminescence excitation spectroscopy. The splitting between the lowest energy heavy‐ and light‐hole excitonic transitions is observed to be only weakly dependent on well width over the range 25–300 A, for sample sets with x=0.56 and x=0.59. Envelope function approximation fitting, based on bulk valence band dispersion calculations which include the strain‐induced interaction with the spin split‐off band, shows this splitting behavior to be a sensitive function of the heterojunction band offset. Conduction band discontinuities, ΔEc, of 0.67ΔEg (x=0.56) and 0.85ΔEg (x=0.59) provide the best fit to these and all higher lying transitions for the full range of structures examined, indicating that poor hole confinement is a limiting factor in practical compressive strain (AlGaIn)P laser device performance.

Si migration effects in GaAs/(Al,Ga)As heterojunction and δ-doped structures

Abstract In order to investigate the Si migration mechanisms thought to be responsible for the anomalous characteristics of a number of heterojunction devices, high depth-resolution SIMS profiling has been used to study these effects in δ-doped and uniformly-doped GaAs samples. For growth at or above ≈ 500°C, surface segregation occurs during the deposition phase, followed by a post-growth diffusion process; the rate of diffusion differs in different Si concentration regimes.