Khalid Mohammed

Resonant tunneling through double barriers, perpendicular quantum transport phenomena in superlattices, and their device applications

New results on the physics of tunneling in quantum well heterostructures and its device applications are discussed. Following a general review of the field in the Introduction, in the second section resonant tunneling through double barriers is investigated. Recent conflicting interpretations of this effect in terms of a Fabry-Perot mechanism or sequential tunneling are reconciled via an analysis of scattering. It is shown that the ratio of the intrinsic resonance width to the total scattering width (collision broadening) determines which of the two mechanisms controls resonant tunneling. The role of symmetry is quantitatively analyzed and two recently proposed resonant tunneling transistor structures are discussed. The third section deals with perpendicular transport in superlattices. A simple expression for the low field mobility in the miniband conduction regime is derived; localization effects, hopping conduction, and effective mass filtering are discussed. In the following section, experimental results on tunneling superlattice photoconductors based on effective mass filtering are presented. In the fifth section, negative differential resistance resulting from localization in a high electric field is discussed. In the last section, the observation of sequential resonant tunneling in superlattices is reported. We point out a remarkable analogy between this phenomenon and paramagnetic spin resonance. New tunable infrared semiconductor lasers and wavelength selective detectors based on this effect are discussed.

Sequential resonant tunneling through a multiquantum well superlattice

We report the observation of two negative conductance regions in the low‐temperature photocurrent‐voltage characteristic of tight‐binding multiquantum well (35 periods) 1‐μm‐thick Al0.48 In0.52As/ Ga0.47 In0.53As superlattices grown by molecular beam epitaxy. The two peaks occur at voltages corresponding to a potential energy drop across the superlattice period equal to the energy differences between the first two excited states and the ground state of the quantum wells. This provides direct evidence of sequential resonant tunneling between the ground and excited states of adjacent wells alternated with intrawell energy relaxation.

Doping interface dipoles: Tunable heterojunction barrier heights and band‐edge discontinuities by molecular beam epitaxy

We have succeeded for the first time in artificially tuning the conduction and valence‐band barrier heights at an abrupt intrinsic semiconductor‐semiconductor heterojunction via a doping interface dipole (DID). This is achieved by means of ultrathin ionized donor and acceptor sheets in situ grown within ≲100 A from the heterointerface by molecular beam epitaxy. In the limit of a few atomic layers separation between the charge sheets this amounts to modify the effective band‐edge discontinuities. A near one order of magnitude enhancement in the photocollection efficiency of an abrupt AlGaAs/GaAs heterojunction has been observed as result of the conduction‐band barrier lowering induced by the DID.

Effective mass filtering: Giant quantum amplification of the photocurrent in a semiconductor superlattice

We report the first observation of an extremely large photocurrent amplification phenomenon at very low voltages in a superlattice of Al0.48In0.52As (35 A)/Ga0.47In0.53As (35 A), grown by molecular beam epitaxy. Responsivities at λ=1.3 μm are as high as 4×103 A/W at 1.4 V corresponding to a current gain of 2×104. Significant gains (≂50) were obtained at voltages as low as 20 mV. The detectivity D* at λ=1.3 μm and 1‐kHz modulation frequency is 1011 (cm Hz1/2/W) at 0.2‐V bias. This effect, which represents a new quantum type photoconductivity, is caused by the large difference in the tunneling rates of electrons and holes through the superlattice barriers, associated with their large effective mass difference (effective mass filtering). Superlattice effective mass filters represent a new class of low voltage, high gain photodetectors. A unique feature, which makes them potentially extremely versatile, is the ability to tune the gain and gain‐bandwidth product over a wide range by varying the superlattice pe...

New avalanche multiplication phenomenon in quantum well superlattices: Evidence of impact ionization across the band‐edge discontinuity

In suitably designed superlattice structures hot carriers in the barrier layers can collide with carriers confined or dynamically stored in the wells and impact ionize them out, across the band‐edge discontinuity. Photomultiplication and spectral response measurements as a function of temperature and chopping frequency in Al0.48 In0.52 As/Ga0.47 In0.53As and AlSb/GaSb superlattices provide strong evidence of this effect. The observed phenomenon is characterized by a large ratio of the multiplication factors for holes and electrons implying that β≫α and can be the basis for a new class of very low noise avalanche photodiodes and solid‐state photomultipliers.

New high‐speed long‐wavelength Al0.48In0.52As/Ga0.47In0.53As multiquantum well avalanche photodiodes

We report the operation of a new long-wavelength (λ=1.3µm) superlattice avalanche photodiode. The p⁺in⁺structure, grown by MBE, contains a 35 period Al<inf>0.48</inf>In<inf>0.52</inf>As (139Å)/Ga<inf>0.47</inf>In<inf>0.53</inf>As (139Å) multiquantum well In the i region. Dc and high frequency multiplications of 65 and 12 respectively have been measured; the dark current at unity gain is 70 nA. High speed of response with full widths at half maximum of 220 ps at a gain of 12 and the absence of tails are demonstrated, indicating that carrier pile-up in the wells is negligible.

Quantum photoconductive gain by effective mass filtering and negative conductance in superlattice pn junctions

Abstract We report a new quantum type photoconductivity, accompanied by a large current gain (>2×10 3 ) in a pn junction with an Al 0.48 In 0.52 As (23A°) Ga 0.47 In 0.53 As (49A°) superlattice in the n layer, forward biased beyond flat band. The high photoconductive gain is caused by the fact that electrons are transported by miniband conduction while holes are mainly localized in the wells (effective mass filtering). When the pn junction instead is reversed biased two peaks are observed in the photo- current vs voltage characteristics. These occur at voltages such that the potential energy difference between two adjacent wells of the superlattice. is equal respectively to the width of the first and second miniband, in excellent agreement with the Dohler-Tsu-Esaki theory. This type of negative differential resistance occurs when the transport mechanism changes from miniband conduction to hopping.

New high speed long wavelength Al 0.48 In 0.52 As/Ga 0.47 In 0.53 As multiquantum well avalanche photodiodes

We report the operation of a new long-wavelength (λ=1.3µm) superlattice avalanche photodiode. The p+in+structure, grown by MBE, contains a 35 period Al 0.48 In 0.52 As (139A)/Ga 0.47 In 0.53 As (139A) multiquantum well In the i region. Dc and high frequency multiplications of 65 and 12 respectively have been measured; the dark current at unity gain is 70 nA. High speed of response with full widths at half maximum of 220 ps at a gain of 12 and the absence of tails are demonstrated, indicating that carrier pile-up in the wells is negligible.