Single pixel isolation of Type II InAs/GaSb strained layer superlattice infrared detectors by chemical etching
REU Student: Travis Nelson
Primary Mentor: Elena Plis
Faculty Mentor: Sanjay Krishna
Introduction
The drive to develop high performance infrared (IR) detectors has been generated primarily by its applications in the military as well as the medical industry including human body detection for surveillance and non-evasive medical imaging and diagnostics. Type II InAs/GaSb strained layer superlattics (SLS), first proposed as an IR detecting material in the 1980’s [14], [15], is now thought as the most promising avenue to further advance IR sensing technology. Although there are other materials, such as mercury cadmium telluride (MCT) or bulk indium antimonide (InSb), which can be used for this same purpose, the InAs/GaSb SLS is theoretically more versatile and efficient than its competitors. Improved properties include; suppressed auger recombination rates due to special separations in the SLS, stronger absorption of normal incident light without the need for gratings used in MCT leading to higher quantum efficiency, a larger electron effective mass than MCT reducing tunneling currents and a more uniform structure [16]. While InAs/GaSb has many expected benefits, it currently has not reached its fullest potential. Part of the reason for the underperformance of the InAs/GaSb SLS detector arises from impurities created when etching the detector material. Complications arise due to the complex chemical composition of the superlattice. A fundamental understanding of SLS structure as well as the mechanisms behind etching is necessary in order to create experiments and improve SLS IR detection.
Background
All objects give off IR radiation, and much of this radiation is in the 3-30µm range that Type II InAs/GaSb SLS’s can detect. Visible light is the most familiar region in the electromagnetic spectrum because it is the wavelength that our eyes can detect. IR radiation, felt as heat can be useful to identify because everything with a temperature above 0 K emits IR radiation. The Infrared region is broken up into different regions, the regions the SLS can detect are mid wavelength infrared (MWIR), long wavelength infrared (LWIR) and very long wave infrared (VLWIR). Higher temperatures give off wavelengths in the MWIR region while the cooler temperatures the VLWIR region. Different gasses give off wavelengths of different regions so dual band, or multiple wavelength detectors can distinguish between their signatures. The human body’s temperature gives off radiation best absorbed by LWIR detectors. Because SLS detectors can be modified to detect each of these regions and as such is extremely versatile. Not only do objects emit IR radiation at different wavelengths but also at different intensities. These characteristics can be used to detect differences in objects with similar chemical compositions especially when they have little or no distinguishable features in the visible light region. The most obvious use of IR detection is as night vision, in the LWIR spectrum, because IR wavelengths are given off by objects at all times, including in the dark. Other important applications of IR detectors are monitoring and recording global temperature profiles, LWIR, missile detection and tracking MWIR, and detection of gas leaks, dual band. For example, exhaled carbon dioxide, while indistinguishable from air, can be detected through a dual-color IR camera as seen in Figure 1. It has also long been known that heat is associated with the health of the human body, LWIR; any deviation from standard temperature can be cause for alarm. The different temperatures can be detected by IR sensors. Cancer also gives off different heat signals and can be detected. All of these applications can be used for early detection of illness and complications as well as non evasive diagnostics. The IR detectors also have a useful tool in astronomy to characterize astronomical features, VLWIR. Planets and other bodies can be characterized through their IR signal such as temperature and composition.
In order to detect IR light a material must respond selectively to specific wavelengths which make up the IR region of the electromagnetic spectrum. This response is an electric current or voltage created by the incident photons. A photon transfers its energy into the valence electrons in the material, and the electrons move up in energy into the conduction band. The difference in energy between the valence and conduction bands is known as the bandgap, and the bandgap energy directly corresponds to the wavelength of light to be detected. The MWIR and LWIR light regions have a wavelength of 3-5µm 8-12µm respectively and are defined by their atmospheric absorption, or the absorption due to water vapor. The absorption for these wavelengths is very small. GaSb and InAs are both III-V semiconductors. Both of these have a crystalline lattice structure known as zincblende as depicted in Figure 2. While both of these semiconductors have the same lattice structure, the bond lengths differ between the two. When the two lattice structures are grown epitaxially, or one on top of the other, the bonds that form do not perfectly matchup. This causes strain in the bonds, a result of the lattice mismatch. The layers of each material are very thin, consisting of several monolayers of GaSb and InAs and switching between the two materials until a desired number of alterations are formed as seen in Figure 3. The name of the material strained layer superlattice is derived from the properties of the total compound. These properties are the strain between the alternating layers and the complex multi layered crystal lattice structure. The thickness of individual monolayers determines the bandgap of the material which determines the wavelength that will excite a valence electron to the conduction band. The SLS structures are typically grown using a process known as molecular beam epitaxy (MBE). This is capable of growing monolayers of each structure accurately with abrupt interfaces.
Fig. 3. Schematic of SLSL IR absorber formed by alternating layers of InAs and GaSb
This leads to the next aspect of the characteristic of the SLS material, which has to deal with the relationship of the bandgap of InAs to GaSb. This bandgap orientation is known as a Type II bandgap. The orientation of our InAs/GaSb superlattice is characterized by two materials that have no overlap of their bandgaps. In our case the valence band of GaSb is higher than the conduction band in InAs which would logically mean that both the conduction and valence band in GaSb is higher than the conduction and valence band in InAs. There are three different known band gap alignments, ranging from type I to type III [9]. The types are characterized by the position of the valance and conduction bands in relation to one another. To review, the valence band is the highest energy band in which electrons resides and the conduction band is the band with higher energy that is the next level up from the valence band. All of the band structures are illustrated in Fig. 4. In a type I superlattice, one of the materials, material A, has a bandgap that resides completely inside the other material, material B. So material A has a valence band with a higher energy than material B’s valence band, and a conduction band with lower energy than material B’s conduction band. Type II’s structure was described earlier and is the orientation of the GaSb/InAs SLS. Type III is similar to type two but differs in that there is some overlap of the two materials bandgaps. While material A has a conduction band higher than material B’s conduction band and a valance band with greater energy than material B’s valance band, in Type III SLS the valence band of material A is not at a higher energy state than material B’s conduction band. As mentioned earlier, the IR light that we are most interested in has wavelengths of 3-12 µm. The individual layers of the superlattice are smaller than this wavelength. This results in a property known as quantum confinement. The importance of this property is that when the material is smaller than the de Broglie wavelength, or the wavelength of photons, the material exhibits different properties than the bulk material. The reason very small materials exhibit differing properties is because of quantum effects. The holes and electrons can tunnel and interact from one material to the other. The combination of quantum confinement due to the thickness of the layers and the Type II band alignment leads to the special detector characteristics of InAs/GaSb SLS. These features allow this material to cater its bandgap to a specific wavelength of IR radiation. This SLS can change its effective bandgap from 3-30 µm which means that both MWIR and LWIR devices can be made out of this material.
Fig. 4 Schematics of various bandalignments [9]
In order to create an imaging device, detector materials must have a specific architecture and receptors that are able to distinguish between different regions of photons. A detector is first grown using MBE. The layers of GaSb and InAs are grown on top of a GaSb substrate wafer. The simplest structure first has a p doping, or region with extra holes, then the SLS structure, followed by an n doped region, or an electron rich region. This structure is referred to as a pin diode, p for the p doped region i for the SLS region and n for the electron rich region. The detector region is only several microns thick. While growth is a very important step in how the detector will perform, many of the inefficiencies arise in the fabrication of the device. Fabrication is necessary in order to isolate regions in the semiconductor for individual detection and metallization. These additions to the Superlattice are necessary to create an image that we can interpret. Pixel isolation is a very important fabrication step. During pixel isolation, regions in the semiconductor are separated from one another. By separating the semiconductor into regions known as pixels, individual signals can be read from each pixel when IR radiation is entering that region. In this way, a picture can be formed by one of two isolation techniques. The first method uses a large region of pixels, each with individual signals. The second system makes use of a single pixel by scanning the pixel across an area and compiling the data received from each point in the scan. These two different ways of obtaining an image also have differences in fabrication. The first process requires many small pixels to be in the same region as each other. This type of detector is known as a Focal Planar Array or FPA. For our FPA detectors a 320 x 256 approximately 20 x 20 µm each is used. On the other hand single, much larger pixels, anywhere from 100 µm to 400 µm, are used for the scanning pattern and are mainly used for research purposes. On the other hand, FPA’s are used in practical applications. In order to isolate these pixels a process known as etching is performed.
Fig. 5. Wafer region with pixel isolation top and side views
There are two types of device definition, wet chemical etching and dry plasma etching. Figure 5 shows the orientation and a rough, enlarged representation of what a semiconductor piece looks like before and after etching. The views are from a top down looking at the wafer, and what the wafer would look like if a pixel were to be looked at from the side after first being cut down the middle. Wet chemical etchants chemically react with the semiconductor, changing the exposed solid to a fluid that detaches and is transported into the solution. The wet plasma etch works by bombarding the surface with ions. Because only a small part of the surface needs to be etched, before etching we need to protect part of the surface. This is done by depositing a layer of a material of photochemical reactive material, photoresist (PR), on the whole semiconductor surface. Next a mask is placed on top of the material. After the mask is in place the sample can be exposed to light, the light only reaches the PR that needs to be removed. This process is known as lithography. There are two basic types of PR used in lithography, positive, which was used for this experiment, and negative. When light hits positive PR, the material becomes soluble in a specific solution while the PR that is not exposed is insoluble. The opposite is true for negative PR. The exposed region is insoluble while the unexposed region is soluble in solution. Positive photoresist is the more commonly used photoresist because it is easier to define small regions while negative photoresist is used because of its better adhesion in on certain materials. Usually only a small area of the photoresist, which means a simpler mask can be patterned for positive photoresist. This is because the mask is composed of a region blocking out light deposited on top of a translucent material. The blocking material must then be removed; it is easier to remove a small portion of this blocking material than a large portion. This means if only 10% of the PR needs to be exposed only 10% of the blocking material needs to be removed for the positive photoresist while 90% would need to be removed for the negative photoresist. The final result is a material protected in specific areas by PR. A final image of a complete, simple detector material with PR can be seen in figure 6. In this image the substrate is first covered with PR and then this PR is removed as described above. Etching can now take place after the PR has been deposited and unwanted PR removed.
Fig. 6. Depiction of deposition and selective removal of positive photoresist