(1) Field of the Invention
This invention relates to photodetectors, particularly to quantum-well integrated photodectors (QWIP) for detecting more than one color.
(2) Brief Description of Related Art
There are many applications for multi-wavelength photodetectors that are simultaneously responsive to two different wavelengths. These applications include missile seekers, infrared cameras for surveillance, target recognition, and environmental monitoring. These wavelengths can be in one of three ranges: near infared (NIR), mid-wavelength infrared (MW), and long-wavelength infrared (LW), Depending on the application, the wavelength pairs required can be NIR-MW, MW-MW, MW-LW, or LW-LW. Implementation of these systems using discrete detectors becomes expensive, bulky and it can result in inferior performances that do not satisfy the system requirements. Monolithically integrated detectors will not only simplify the signal processing aspect of the system but also results in critical performance enhancements. Significant flexibility and performance improvements can be achieved by adding more detectors to cover a broader wavelength.
Multi-wavelength detectors require integration of detectors with different vertical physical dimensions and different responsivities within their individual spectral bands. There are two approaches of monolithically integrating two detector materials on a single substrate. First, is the widely used vertical integration, in which the material for the two wavelengths are grown vertically in one growth sequence, with a thin etch stop layer grown between each QWIP structure. This approaches yield the best material properties because both device layers are grown together without growth interruptions. The typical thickness of the standard QWIP material structures that are commonly used m and thickness of the vertically integrated dual wavelength μ are about 2.0 m. Fabrication of QWIP based focal plane arrays structure is greater than 4.0 require etching of mesas through these structures to isolate each of the tens of thousands of pixels in a typical array. While the thickness of the single wavelength structures is manageable for device processing, the multi-wavelength structures are virtually impossible to fabricate because of the excessively tall mesas. The tall mesas will result in lower device fabrication yields. Additionally, the wafer growth cost is increased because of the prolonged growth duration. Further, the non planarity of the structures will make fabrication of four different gratings for coupling the radiation difficult.
The drawback of the fabrication process complexity for the vertical integration of two-color QWIPs can be alleviated by lateral integration. The integration of two colors (MWIR and LWIR) by selective epitaxy of MW and LW structures on InP substrate yields manageable total thickness of the layers for device processing, but requires two growth sequences as disclosed in U.S. Pat. No. 6,407,439 B1 However, both approaches are limited to integration of two QWIP structures having response in MW-MW or LW-LW or MW-LW region. As a result of this serious problem, in this invention a thinner layer structure designs is proposed that will be easier to fabricate. The approach includes using novel quantum well material combinations to achieve two wavelength windows using thicknesses that are comparable to those of single wavelength designs.
An object of this invention is to integrate vertically a quantum-well photodetector using thinner layer structure which is easier to fabricate. A second object of this invention is to achieve two wavelength windows using thicknesses that are comparable to those of single wavelength designs.
These objects are achieved by bandgap engineering in a single structure to extend the design of standard QWIPs to one that contains a period with two different types of quantum wells, i.e. a multi-wavelength structure. Secondly, the new QWIP structure contains a'sequence of barrier-well-subwell-well-barrier layers.
a shows a standard GaInAs quantum well structure with AlInAs barriers;
a shows GaInAs quantum well structure with InAs sub-well. Total thickness of the well 46 Å, sub-well thickness 24 Å;
a shows a layer structure of a single period for a 4-μm/5-μm dual-wavelength quantum well infrared photodetector that uses InAs sub-wells;
a shows a typical GaAs based material structure of the combined MW-LW detector;
a shows a typical material structure of the dual LW-IR wavelength QWIP;
a shows mixed well design to minimize the dark current with AlGaAs barrier, Ga0.7In0.3 As well, GaAs mini-well;
a and 1b show the energy levels and calculated photoresponse of a single well of a standard barrier-well-barrier type QWIP. The present invention contains a sequence of barrier-well-subwell-well-barier layers as shown in
One such design and the photoresponse are illustrated in
Simulations were performed extensively to investigate the effect of the material system, location, number, and thickness of the sub-well response of the novel detector structure. First, the number of standard wells was varied while keeping number of the sub-well to two and total number of wells constant (20).
The third example of the simulation of the QWIP is changing the material structure to GaAs/AlGaAs.
Further, simulations were performed by varying the number, and thickness of sub-wells, and the ratio of sub-wells to standard wells. For example, the structure shown in
An alternative to a two-wavelength detector is one that has a wider spectral response peak.
A second example of a typical QWIP material structure for a 8-μm/12-μm dual wavelengths uses GaInAs as well material (with and without sub-well) and AlGalnAs as the barrier material. Each set of two wells and barriers are optimized to give the required two-color operation. The addition of the sub-well reduces the lowest wavelength possible for the material system and structure chosen, compared to the structure without the sub-well. The structure can be designed to place the lowest energy level of the quantum well in the sub-well. Since the energy level is pinned to that position; this allows the independent adjustment of the upper level of the transition by changing the well width.
The final structure investigated was the integration of two such dual-color QWIPs shown in figures in 3a, 3b, 9a and 9b. The simulated spectral response of the integrated gave four absorption peaks at 4.0, 5.0, 8.0, and 12.5 μm wavelength. The material system contains AlinAs and AlGaInAs as barriers, a GaInAs well and an InAs sub-well. In order to evaluate the spectral response of novel designs of the multi-wavelength detectors, a vertically integrated TWO-dual wave length structures (mid-wave/mid-wave and long-wave/long-wave) was grown on InP substrate.
In a standard QWIP, the barrier is undoped and the well is doped. The doped well layers can be n or p-type. The doping can be uniform or delta/spike/planar doped. The spike doping can be Si, Sn, Te and Be, C for n and p-type QWIPs. Alternately, the barrier layers can be delta/spike/planar doped and provides donor or acceptor supply layer for the carriers in the well (undoped) of the QWIP. The spike doping or delta or planar doping provides two-dimensional electron or hole gas in the well and increases the mobility, which improves the performance of the QWIP. The sub-well can be narrow bandgap semiconductor or its quantum dots (e.g. InAs quantum dot in GaInAs well). Typical thicknesses of the well and the subwell are 2.0-10 mn and 0.5-3.0 nm respectively.
The combination of barrier/well materials in the QWIP structure can be extended to other semiconductor. The combination of the barrier/well can be: GaAs/AlGaAs, GaAs/GaInP, GaAs/AlAs, GaInAs/AlGaAs (AlAs, GaInP); GaInAs/AlInAs, GaInAs/AlGaAsSb, GaInAs/AlAsSb, GaAsSb/AlGaAsSb, GaInAs/AfnAsSb (InP), InP/AlInAs (AlGaAsSb, AlAsSb, AlGaAsSb, AInAsSb), InAs/AlGaAsSb(AlSb, AlAsSb, AlGaSb), InAs/AlGaInSb, InSb/AlInSb, GaN/AlGaN, GaInN/AlGaN, etc. and any other combination (including Thallium compounds) of binary, ternary, quadranary III-V semiconductor. QWIP can be extended to IV-IV semiconductor (Si, Ge, Sn, C), II-VI semiconductors (ZnSe, ZnS, CdTe, CdS, etc) or combinations of Ill-V and IV-IV (e.g. GaP/Si) or IV-IV and II-VI (e.g. ZnS/Si or III-V and II-VI (e.g. ZnSe/GaAs). The sub-well can be narrow bandgap semiconductor such as InAs, InSb, TlAs, TlP, TlSb, Ge, GeSn, etc. The substrate can be Si, GaAs, GaN, SiC, InP or other substrates on which the QWIP heterostructure is transferred by bonding/lift-off/growth. The QWIP structure can be can be grown via the group consisting of MBE/CBE/MEE/GSMBE/VPE/OMVPE/UHVCVD etc.
Leakage Current Reduction:
In the second embodiment of the invention is to implement a novel device structure to reduce leakage currents in the quantum well infrared photo-detectors (QWIPs). The standard QWIPs suffer from high leakage currents and poor leakage current uniformity, which results in degradation of the response. For wavelengths greater than 8.0 μm, the leakage current is worst because of smaller conduction band discontinuity for both GaAs and InP based devices. The leakage currents in QWIPs can be divided into two types: 1) process induced and 2) dark current. Careful processing of the devices can minimize the process induced leakage current and the uniformity. The origin of dark current in the QWIPs is due to: 1) phonon excitation, 2) thermionic and 3) defect-assisted tunneling. The dark currents due to the last two mechanisms can be minimized by proper design of the quantum well structure. Lower dark current enable background limited infrared photodetector (BLIP) operation at higher temperatures. Overall the leakage currents are detrimental to the detectivity of the detector. Various techniques have been proposed to reduce the leakage currents such as inserting a blocking layer in GaAs/AlGaAs QWIPs, (C. S. Wu et al., IEEE Tran. ED, Vol. 39, p. 234, 1992). In this structure, a thicker barrier of similar conduction band discontinuity was used, which is not sufficient to reduce leakage at higher temperatures (>40 K). In this invention, three approaches are proposed to reduce the leakage current in the QWIPs. First, is to add in the collector of the QWIP, an additional barrier of AlAs (AlSb, and AlAsSb) having large conduction band discontinuity and thickness of 2-5 nm to reduce the dark current. In this design, the lower energy electrons do not overcome the large conduction band discontinuity. The advantage of this approach is that, it requires one extra barrier and easy to implement.
The second approach is to add a mini-well in the barrier of the QWIP, which enables electrons to tunnel through the barriers. The mini-level is designed to have a single state that is resonant with the upper state of the GaInAs well. This design shown in the
The third approach is to use step barriers as shown in the
The advantages claimed for this invention are:
While the preferred embodiments of the invention have been described, it will be apparent to those skilled in the art that various modifications may be made in the embodiments without departing from the spirit of the present invention. Such modifications are all within the scope of this invention.