JP2017147324A - Infrared detector and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an infrared detector having a wide detection wavelength band.SOLUTION: An infrared detector comprises: a quantum dot layer 13 including a plurality of quantum dots; first and second quantum well layers 12 and 14; first and second barrier layers 11 and 15; and a pair of intermediate layers 10. The infrared detector is arranged to utilize the inter-level transition of electrons 17 in the plurality of quantum dots based on light to be detected. To at least one of the first and second quantum well layers 12 and 14, the first and second barrier layers 11 and 15, and the quantum dot layer 13, a variation is imparted. Thus, the infrared detector has a variation depending on a desired detection band in the excitation level EL1 of electrons 17 in each quantum dot.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a quantum dot infrared detector and a method for manufacturing the same.

  In recent years, there has been an increasing demand for infrared detectors for the purpose of heat detection and concentration measurement of carbon dioxide and air pollutants. There are a plurality of candidates for the material and structure of the infrared detector, one of which is a quantum dot infrared detector (QDIP) in which a semiconductor quantum dot is included in the light absorption layer.

  QDIP has a structure in which a quantum dot is three-dimensionally surrounded by a semiconductor having a larger band gap than the material constituting the quantum dot. Electrons and holes are strongly confined in the quantum dot region (typically a size of several tens of nanometers). As a result, discrete energy levels are formed in the quantum dots. QDIP can detect infrared rays having energy corresponding to the energy difference between subbands by using a plurality of electronic subband levels in the conduction band among these levels.

Since QDIP uses discrete intersubband transitions, each quantum dot has a discrete detection wavelength band. However, in practice, since the detector includes a large number of quantum dots, the detection wavelength band has a spread corresponding to an inevitable variation (non-uniform spread) in the size and shape of the quantum dots. For example, if there are 10 ¹¹ quantum dots per 1 cm ² and the light receiving surface of the detector is 300 μm square (the area is 9 × 10 ⁻⁴ cm ² ), it includes 9 × 10 ⁷ quantum dots. It is. This non-uniform spread is reflected in the detection wavelength band of the entire detector.

By the way, as a performance index of an infrared detector including QDIP, there is a specific detectability. The specific detectability D ^* is defined by the following equation 1.

  Here, R is the light receiving sensitivity (unit is A / W), S is the light receiving area, Δf is the bandwidth, and in is the noise current density per unit bandwidth (1 Hz) (unit is A / √Hz).

The specific detectability D ^* indicates how much AC S / N is present when there is a 1 W optical input. As is clear from Equation 1, the specific detectability D ^* is standardized by the light receiving area and the bandwidth, and can be compared by different elements. It can be said that the higher the specific detectability D ^* , the better the infrared detector.

As a technique for increasing the specific detectability D ^{*, a structure in} which a quantum dot layer (and a quantum well layer surrounding the quantum dot layer) is sandwiched between barrier layers made of AlAs or AlGaAs having a wider band gap than those is known. (Patent Document 1, Non-Patent Document 1). According to this structure, the electrons are strongly confined by the quantum dots, and the electron transition probability is increased. As a result, a high specific detectability D ^* is obtained. For example, Non-Patent Document 1 shows that in the actual structure, the specific detectability D ^* is increased by about 10 times compared to the case without the proximity barrier layer.

  In addition to Patent Document 1 and Non-Patent Document 1, quantum dot infrared detectors having a barrier layer are disclosed in Patent Documents 2 to 4, for example.

JP 2009-066512 A JP 2012-195333 A JP 2012-109434 A JP 2012-023307 A

A.V.Barve et al., Applied Physics Letters 99, 2, 191110-1 to 191110-3 (issued 2011) A. Gomyo et al., Phys. Rev. Lett. 72 pp. 673-676 (1994)

  One of the features of QDIP is that the detection wavelength is a narrow band. Then, it is known that the detection center wavelength of QDIP can be set to a desired value by changing the quantum dots included in the light absorption layer or its peripheral structure.

  However, depending on the application of the infrared detector, it may be preferable that the detection wavelength has a wide band. However, the technique for obtaining a wide detection wavelength band is in fact not fully studied.

  Therefore, an object of the present invention is to provide an infrared detector having a wide detection wavelength band and a method for manufacturing the same.

  According to one aspect of the present invention, a quantum dot layer including a plurality of quantum dots, a pair of quantum well layers sandwiching the quantum dot layer and having a larger band gap energy than the quantum dot layer, and the pair of quantum wells A pair of barrier layers having a band gap energy larger than that of the pair of quantum well layers and a pair of intermediate layers sandwiching the pair of barrier layers, and having electrons based on excitation light that is detection target light. An infrared detector using a transition between a ground level and an excited level in the quantum dot, wherein at least one of the quantum well layer, the barrier layer, and the quantum dot layer has a structure and a composition At least one of the quantum dots is provided with a variation, whereby at least one of the electron excitation level and the ground level of each of the quantum dots has a size variation corresponding to a desired detection bandwidth. Infrared detector according to claim Rukoto is obtained.

  According to another aspect of the present invention, a step of forming a quantum dot layer including a plurality of quantum dots and a pair of quantum well layers having a band gap energy larger than that of the quantum dot layer sandwiching the quantum dot layer are formed. Forming a pair of barrier layers having a band gap energy larger than that of the pair of quantum well layers, and forming a pair of intermediate layers sandwiching the pair of barrier layers. A method of manufacturing an infrared detector using a transition between a ground level and an excitation level in the quantum dot of electrons based on excitation light that is detection target light, the method comprising: The at least one of the barrier layer and the quantum dot layer is given a variation in at least one of its structure and composition, and the excitation level of each of the quantum dots and Bottom level of the at least one method of an infrared detector, characterized in that to have a variation in size corresponding to the desired detection bandwidth is obtained.

  According to the present invention, an infrared detector having a wide detection wavelength band can be obtained.

It is sectional drawing which shows the basic structure of the infrared detector by Embodiment 1-5 of this invention. It is sectional drawing which shows the basic structure of the principal part of the infrared detector by Embodiment 1-5 of this invention. It is an energy band figure for demonstrating operation | movement of the infrared detector by Embodiment 1-5 of this invention. It is a figure for demonstrating the infrared detector by Embodiment 1, 2 of this invention, (a) is an energy band figure, (b) is a figure which shows the sensitivity with respect to a detection wavelength band. It is a figure which shows the example of a measurement of the wavelength response spectrum of the infrared detector by Embodiment 1 of this invention. It is a figure for demonstrating the infrared detector by Embodiment 3 and 4 of this invention, (a) is an energy band figure, (b) is a figure which shows the sensitivity with respect to a detection wavelength band. It is a figure showing the photo-luminescence light emission characteristic from a quantum dot layer, (a) shows the characteristic of the infrared detector by a comparative example, (b) shows the characteristic of the infrared detector by Embodiment 3 of this invention. It is the figure which represented typically the dispersion | variation in the height of the quantum dot in the infrared detector by Embodiment 3 of this invention. It is a schematic diagram showing the light reception characteristic of the infrared detector by Embodiment 5 of this invention. It is a figure for demonstrating the infrared detector by the modification which combined Embodiment 1-5 of this invention, (a) is an energy band figure, (b) is a figure which shows the sensitivity with respect to a detection wavelength band.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a cross-sectional view showing the structure of an infrared detector 100 according to an embodiment of the present invention. 2 is a cross-sectional view showing the structure of the light absorption layer 4 in the infrared detector 100 shown in FIG. In these drawings, the ratio of the thickness dimension of each layer, the ratio of the diameter dimension of the quantum dots, and the like are not drawn accurately for convenience of explanation.

  As shown in FIG. 1, a buffer layer 2 is formed on a semiconductor substrate 1. The buffer layer 2 is made of the same semiconductor material as that of the semiconductor substrate 1. A lower contact layer 3 is formed on the buffer layer 2. The lower contact layer 3 is composed of an n-type semiconductor as a main material. The lower contact layer 3 may be directly formed on the semiconductor substrate 1 without the buffer layer 2 interposed therebetween. A light absorption layer 4 and a lower electrode 7 are formed on the lower contact layer 3.

  If the semiconductor substrate 1 is n-type, the lower electrode 7 may be formed so as to be in contact with the semiconductor substrate 1. In addition, the quantum dot layer 13 is included in the light absorption layer 4, and details will be described later with reference to FIG.

  An upper contact layer 5 is formed on the light absorption layer 4. The upper contact layer 5 is composed of an n-type semiconductor as a main material. The light absorption layer 4 and the upper contact layer 5 are partially removed by an etching process or the like, and a part of the upper contact layer 5 is covered with the upper electrode 6.

  Under the condition that an appropriate voltage is applied between the upper electrode 6 and the lower electrode 7, when the infrared ray X is incident from the upper part of the paper as shown by the arrow, the light absorption layer 4 emits infrared rays having a wavelength corresponding to the structure. The electrons are absorbed to excite electrons, and as a result, a photocurrent flows between the upper electrode 6 and the lower electrode 7.

  Referring to FIG. 1 and FIG. 2 together, the light absorption layer 4 has a quantum dot layer 13 composed of a plurality of quantum dots 131 and a buried layer 132, and a quantum dot layer 13 sandwiched between the quantum dot layer 13 and the band. The first quantum well layer 12 and the second quantum well layer 14 having a large gap energy, and the first quantum well layer 12 and the second quantum well layer 14 are sandwiched between the first quantum well layer 12 and the second quantum well layer 14. It has the 1st barrier layer 11 and the 2nd barrier layer 15 whose band gap energy is larger than the quantum well layer 14, and the pair of intermediate | middle layer 10 which pinches | interposes the 1st barrier layer 11 and the 2nd barrier layer 15 ing.

  Furthermore, in this example, the series of laminated bodies described above are repeatedly laminated about 10 times. Thereby, the infrared absorption efficiency in the light absorption layer 4 is improved. However, in FIG. 1, the illustration is simplified and three repetitions of lamination are shown.

[principle]
FIG. 3 is an energy band diagram for explaining the operation principle of the present embodiment, and shows the conduction band energy when the bias of the infrared detector 100 is applied. In FIG. 3, the vertical direction in FIG. 1 corresponds to the left-right direction, and the upper side in FIG. 1 corresponds to the right side in FIG. Strictly speaking, the electron energy band of the quantum dot has a three-dimensional structure, but is represented by one dimension as an approximation.

  As is clear from FIG. 3, the energy of the quantum dot layer 13 is the lowest, the energy of the intermediate layer 10 is higher than that of the first quantum well layer 12 and the second quantum well layer 14, and the first barrier layer 11 The energy of the second barrier layer 15 is the highest. With such a structure, the ground level EL1 and the excitation level EL2 of the electrons 16 confined in the region centering on the quantum dot layer are formed.

  When infrared rays corresponding to the energy difference between the excitation level EL2 and the ground level EL1 are incident in the state where the electrons 16 are present in the ground level EL1, the energy of the infrared rays is absorbed and the electrons 16 are excited from the ground level EL1. Excited to position EL2. A bias voltage is applied between the upper electrode 6 and the lower electrode 7, whereby the electrons 16 can tunnel through the second barrier layer 15 and detect infrared rays as a change in current.

  Although FIG. 3 shows a band structure for one quantum dot, a large number of quantum dots are included in one infrared detector including this embodiment. The size and shape of these quantum dots vary by a certain width, which is also called non-uniform spread. Due to this non-uniform spread, the ground level EL1 and the excitation level EL2 of the quantum dots vary, and these variations determine the detection wavelength band of the infrared detector.

  In general, when trying to form high-quality quantum dots, the sizes and shapes tend to be uniform as a result, so it is difficult to intentionally increase the non-uniform spread. As a result, infrared detectors have narrow band operation and are one of the main features of QDIP.

[Embodiment 1]
[Constitution]
The infrared detector according to Embodiment 1 of the present invention basically has the structure described above with reference to FIGS. 1 and 2 (infrared detector 100).

  However, in the present embodiment, structural variations are intentionally given to the first barrier layer 11 and the second barrier layer 15, and as a result, the excitation levels of the electrons of each of the plurality of quantum dots 131 vary. Is granted. This will be described below with reference to FIGS. 4 (a) and 4 (b).

  The thickness of the first barrier layer 11 and the second barrier layer 15 is set to a thickness (average value) of about 1 nm. Usually, a semiconductor thin film layer that needs to be controlled in thickness on the order of nanometers is manufactured using a metal organic chemical vapor deposition method, a molecular beam epitaxial method, or the like. However, even if these precise manufacturing techniques are used, it is difficult to manufacture a semiconductor layer having a flat film thickness at a completely uniform atomic layer level, and there is always a variation of at least about one atomic layer. Yes.

  This inevitable variation in thickness becomes a thickness variation that cannot be ignored when the AlGaAs layer is thin as in the present embodiment. This means that a barrier layer having a thick region and a thin region is formed on the wafer. As a result, the infrared detector according to the present embodiment gives variation to the excitation level EL2 of the quantum dots.

  For example, in the present embodiment, the first barrier layer 11 and the second barrier layer 15 are AlGaAs, and the thickness of one atomic layer is about 0.3 nm. Therefore, the thickness of the barrier layer is at least ± 30% variation.

  Further, when the barrier layer is formed by crystal growth, the thin barrier layer has an effect of causing further generation of a surface step. Usually, discontinuity of the crystal structure due to bonding of different materials occurs at the junction growth interface of different materials, and dislocations or steps (steps) may occur. However, when the growth layer thickness is thick, these dislocations and steps are eliminated and the flatness is restored as the growth layer thickness increases.

  On the other hand, when the thickness of the growth layer is thin as in this embodiment, the flatness of the surface is not sufficiently recovered in the growth process of the growth layer until the thin thickness is reached. Therefore, a variation is given to the thickness of the barrier layer. As a result, variations are imparted to the excitation level EL2.

  Furthermore, the step of the growth layer surface, that is, the first barrier layer 11 and the second barrier layer 15 can also be reduced by lowering the substrate temperature within a range where the crystal growth is not broken during growth, or by reducing the As pressure. Variation in the thickness of the substrate can be increased. As a result, even greater variations can be imparted to the excitation level EL2.

  By the way, since the wave function of the ground level EL1 is strongly confined in the quantum dot, it is hardly affected by the outside of the quantum dot. That is, the influence of the variation in the thickness of the barrier layer is small, and the variation in the ground level EL1 is not different from the value determined by the variation in the quantum dots. On the other hand, the excitation level EL2 is greatly affected by variations in the thickness of the barrier layer since the wave function spreads to the periphery of the barrier layer. As a result, as shown in the lower graph of FIG. 4B, the excitation level EL2 is much wider than the amount determined by the variation of the quantum dots.

  FIG. 4B schematically shows the wavelength response of the infrared detector. In FIG. 4B, the spectrum indicated by the broken line in the upper graph indicates the wavelength response of the infrared detector according to the comparative example. In this comparative example, the barrier layer has a thickness of 2 nm and the thickness variation is ± 15%. On the other hand, the spectrum shown by the solid line in the lower graph in FIG. 4B shows the wavelength response of the infrared detector 100 according to the present embodiment. As described above, in the infrared detector 100 according to the present embodiment, the barrier layer has a thickness of 1 nm and the thickness variation is ± 30%. As is clear from FIG. 4B, the infrared detector 100 according to the present embodiment has a wider detection wavelength band due to a larger variation in the excitation level EL2 than in the comparative example.

  Next, a more specific structure of the infrared detector 100 according to the present embodiment will be described.

Referring to FIG. 2, the intermediate layer 10 _consists of Al _{y Ga 1-y} As. On the intermediate layer 10, a first barrier layer 11 made of Al _x Ga _1-x As having an average thickness d (nm) is formed. A first quantum well layer 12 made of GaAs having an average thickness of about 1 to 2 nm is formed on the first barrier layer 11. A quantum dot layer 13 having a thickness of about 5 nm is formed on the first quantum well layer 12. The quantum dot layer 13 includes a plurality of quantum dots 131 made of InAs with a height of about 4 nm, and a buried layer 132 made of In _0.15 Ga _0.85 As with a thickness of about 5 nm filling between the plurality of quantum dots 131. It is constituted by. The buried layer 132 does not necessarily have to be thicker than the quantum dots 131 as in this example, and may have a thickness equal to the height of the quantum dots 131.

A second quantum well layer 14 made of GaAs having an average thickness of about 1 to 2 nm is formed on the quantum dot layer 13. A second barrier layer 15 made of Al _x Ga _1-x As having an average thickness d (nm) is formed on the second quantum well layer 14. On the second barrier layer 15, the intermediate layer 10 made of Al _y Ga _1-y As is formed. The light absorption layer 4 shown in FIG. 1 is configured by repeatedly stacking this series of laminated structures, for example, 10 times.

FIG. 5 illustrates the intermediate layer 10 (composition Al _y Ga _1-y As), the first barrier layer 11, and the second barrier layer 15 (composition Al _x Ga _1-x As, thickness d (nm)). Wavelengths of the structure A as a comparative example in which x = 0.18 and y = 0.03 are set and the thickness d = 1 nm and the structure B according to the present embodiment in which the thickness d = 2 nm are set. The measurement result of a response spectrum is shown.

In the structure A as a comparative example, the detection peak wavelength is in the vicinity of 5.5 μm, and the full width at half maximum is about 1.2 μm. On the other hand, the structure B according to the present embodiment is more sensitive to the longer wavelength side than the structure A, and the full width at half maximum is expanded to about 2 μm. Further, the peak value of the non-detectability D ^* of the structure B according to the present embodiment is lower than that of the structure A, but is only about 20% lower.

  From this result, the following can be understood. That is, in the structure A as a comparative example in which the thickness of the barrier layer is 2 nm, the variation in the thickness of the barrier layer at the atomic layer level is the same as the non-uniform spread inherent in the quantum dots, or It is as follows. For this reason, it is thought that it is not visible as the expansion of the detection wavelength band. On the other hand, in the structure B in which the thickness of the barrier layer is 1 nm according to the present embodiment, the variation with respect to the thickness of one atomic layer is twice that of the structure A. Along with this, the variation of the excitation level EL2 increases. As a result, it is considered that the detection wavelength band has expanded.

  As in the present embodiment, when the quantum dot layer is made of InAs, the barrier layer is made of AlGaAs, and the quantum well layer is made of GaAs, the average thickness of at least one of the barrier layer and the quantum well layer is It is preferable that it is 1/4 or less of the height of a dot.

In addition, regarding the Al composition x of the barrier layer made of Al _x Ga _1-x As and the Al composition y of the intermediate layer made of Al _y Ga _1-y As, if the Al composition x of the barrier layer is too large, the quantum dots Since the electrons are strongly confined and the crystal quality is lowered, it is not suitable for the operation of the infrared detector. Therefore, the Al composition x of the barrier layer is preferably 0.20 or less. In the structure B of FIG. 5, the thickness of the barrier layer is 1 nm and the Al composition x = 0.18. If the Al composition x of the intermediate layer is too close to the Al composition x of the barrier layer, the effect as a thin barrier layer is reduced. Therefore, it is desirable that the difference between the Al composition x of the barrier layer and the Al composition y of the intermediate layer is 0.1 or more (xy ≧ 0.1). In the structure B of FIG. 5, y = 0.03.

In this way, the desired detection wavelength band of the quantum dot infrared detector can be achieved while suppressing the reduction of the specific detectability D ^* by changing the composition of the intermediate layer and the barrier layer while making the barrier layer about 1 nm thick. Can be obtained.

  In the present invention, the average thickness of at least one of the barrier layer and the quantum well layer is preferably 1 nm or less. In the case where the average thickness of at least one of the barrier layer and the quantum well layer is not 1 nm or less, it is preferable that the average thickness variation is 4 atomic layers or more.

[Production method]
Next, with reference to FIG. 1 and FIG. 2, the manufacturing method of the infrared detector by Embodiment 1 of this invention is demonstrated.

A GaAs substrate having a (001) plane orientation is prepared as the semiconductor substrate 1 and introduced into the molecular beam epitaxial apparatus. After removing the thin natural oxide film on the substrate surface, a buffer layer 2 made of the same GaAs as that of the semiconductor substrate 1 is laminated to a thickness of about 500 nm. The pressure of As at the time of crystal growth was 1 × 10 ⁻⁵ Torr.

Subsequently, an n-type lower contact layer 3 made of GaAs having a thickness of 500 nm and doped with Si atoms at a concentration of about 2 × 10 ¹⁸ cm ⁻³ , and an i-type made of Al _y0.03 Ga _0.97 As _{having a} thickness of 50 nm. The intermediate layer 10 is laminated.

Further, a first barrier layer 11 made of Al _0.18 Ga _0.82 As having an average thickness of about 1 to 2 nm, a first quantum well layer 12 made of GaAs having an average thickness of about 1 to 2 nm, and a plurality of layers. The quantum dot layer 13 composed of the quantum dots 131 and the buried layer 132 is stacked in this order.

  The plurality of quantum dots 131 are formed by supplying InAs corresponding to a thickness of about 2 to 3 atomic layers. At this time, due to the strain generated due to the difference in lattice constant between InAs and GaAs directly thereunder, InAs grows three-dimensionally in an island shape, and quantum dots 131 are formed. This crystal growth mode is called SK (Stranski-Krastanov) mode. In the SK mode, a plurality of quantum dots 131 are formed simply by supplying a material. Therefore, the quantum dots formed in the SK mode are also called self-formed quantum dots.

Each quantum dot 131 has a diameter of about 20 to 30 nm, a height of about 4 nm, and a number density per square centimeter of about 5 × 10 ¹⁰ .

  In order to operate as an infrared detector, each quantum dot 131 needs to contain one electron in advance. In order to satisfy this, Si, which is an n-type impurity, is doped with a surface density comparable to that of the quantum dots.

Subsequently, the quantum dot layer 13 is formed by forming the buried layer 132 made of In _0.15 Ga _0.85 As having a thickness of about 5 nm. If necessary, the top of the quantum dots 131 is evaporated by a process called an indium flash method so that the height of the quantum dots 131 is less than or equal to the thickness of the buried layer 132, thereby providing a plurality of quantum dots 131. May be arranged so as not to jump out to the second quantum well layer 14.

  Thereafter, a second quantum well layer 14 made of GaAs having an average thickness of about 1 to 2 nm is stacked.

Subsequently, a second barrier layer 15 made of Al _0.18 Ga _0.82 As having an average thickness of about 1 to 2 nm is stacked, and i-type Al _y0.03 Ga _{0.97 having} a thickness of 50 nm is further stacked. The intermediate layer 10 made of As is laminated again.

  According to the above procedure, the first barrier layer 11, the first quantum well layer 12, the quantum dot layer 13, the second quantum well layer 14, the second barrier layer 15, and the intermediate layer 10 are repeatedly stacked about 10 times. Then, the light absorption layer 4 is formed.

Finally, an n-type upper contact layer 5 made of GaAs having a thickness of 200 nm and doped with Si atoms at a concentration of about 2 × 10 ¹⁸ cm ⁻³ is laminated.

  In the above manufacturing method, quantum dots and quantum wells and their peripheral structures are formed by the MBE method. However, the present invention is not limited to this method, and other crystal growth methods such as metal organic chemical vapor deposition are used. May be.

  Subsequently, the upper contact layer 5, the light absorption layer 4 and a part of the lower contact layer 3 are selectively etched using ultraviolet lithography, wet etching, or dry etching technology. As a result, a part of the surface of the lower contact layer 3 is exposed.

  The structure separated by this selective etching becomes the infrared detector 100. The size of the light receiving surface of the detection element varies depending on the application, but is typically about 20 μm square to 500 μm square. The infrared detector may be composed of only this one element, or may be an array in which such elements are arranged in one row or two-dimensionally.

  Next, an upper electrode 6 and a lower electrode 7 made of AuGe / Ni / Au are formed on the upper contact layer 5 and the lower contact layer 3 by a lift-off method, respectively. The lift-off method includes processes such as lithography, metal deposition, and resist stripping.

  Through the above steps, the basic configuration of the infrared detector according to the first embodiment of the present invention is completed.

[Embodiment 2]
Next, Embodiment 2 of the present invention will be described.

  In the first embodiment, by setting the average thickness of the barrier layer and the quantum well layer to about 1 nm or less, by providing the barrier layer and the quantum well layer with a structure in which the thickness variation is relatively negligible, Intentionally created variations in the effective thickness of the barrier and quantum well layers. As a result, variations in excitation levels are created, and as a result, the detection wavelength band of the infrared detector is widened in the first embodiment. On the other hand, in the present embodiment, the same effect is obtained by at least one of the first and second quantum well layers 12 and 14 (FIG. 2) and the first and second barrier layers 11 and 15 (FIG. 2). By intentionally creating variations in the composition, the detection wavelength band of the infrared detector can be widened.

  As described in the first embodiment, a semiconductor thin film layer that needs to be controlled in thickness on the order of nanometers is usually manufactured using a metal organic chemical vapor deposition method, a molecular beam epitaxial method, or the like. When crystal growth is performed under normal conditions by these techniques, the composition of the grown layer produced is almost uniform even if the thickness of the grown layer is reduced.

  However, in the case of a compound semiconductor composed of a plurality of group III elements, a plurality of types of group III elements can be obtained by increasing the pressure of group V during crystal growth or significantly lowering the growth temperature than usual. Are known to be regularly arranged (Non-Patent Document 2).

  When the first group III element is IIIA and the second group III element is IIIB, the group III element is usually, for example, “... IIIA / IIIA / IIIB / IIIA / IIIB / IIIB ...” In addition, there is no regularity because it randomly enters the Group III site.

  However, if manufacturing is performed with the above conditions, that is, the pressure of the group V during crystal growth is increased or the growth temperature is greatly lowered than usual, for example, "... IIIA, IIIB, IIIA, IIIB" “IIIA, IIIB...”, IIIA and IIIB are arranged alternately and regularly.

  In addition, this regular arrangement structure of group III elements is not formed uniformly over the entire surface of the growth layer, particularly when the molecular beam epitaxial growth method is used. Is done.

  In addition, when the group III elements are regularly arranged, there is a phenomenon that the band gap energy is reduced as compared with a normal case. That is, variation in the band gap energy of the barrier layer and the quantum well layer occurs due to the local regular arrangement of group III elements. As a result, it is possible to cause variations in the excitation levels of the quantum dots.

  Furthermore, this regular arrangement structure of group III elements tends to be generated with atomic steps (steps) on the surface of the crystal growth layer as nuclei. Therefore, when a sufficiently thin barrier layer or quantum well layer is grown on the surface on which many atomic order steps exist as described in the first embodiment, the group III element is formed in more places. A regular array structure is formed. As a result, larger excitation level variations can be caused.

  Specifically, a semiconductor wafer for an infrared detector is manufactured according to the following steps. The manufacturing process is the same as that described in the first embodiment except for the crystal growth process of the barrier layer or the quantum well layer. For this reason, only the manufacturing conditions of the quantum well layer will be described below.

  The steps before the crystal growth of the first quantum well layer 12 (FIG. 2) made of InGaAs and the steps after the crystal growth of the second quantum well layer 14 (FIG. 2) also made of InGaAs were performed. In the same manner as in Embodiment 1, crystal growth of the semiconductor wafer was performed.

Immediately before each growth of the first quantum well layer 12 and the second quantum well layer 14, the As pressure was raised to 2.5 × 10 ⁻⁵ Torr and the substrate temperature was lowered to 370 ° C. Each crystal growth of the first quantum well layer 12 and the second quantum well layer 14 was performed.

  After each growth of the first quantum well layer 12 and the second quantum well layer 14, the crystal growth is performed after returning to the growth conditions before the growth of the first quantum well layer 12 and the second quantum well layer 14. Continued.

  In addition, growth other than the 1st, 2nd quantum well layers 12 and 14 followed the process similar to Embodiment 1, and manufactured the infrared detector.

  As for the wavelength response of the infrared detector manufactured by the above-described process, it was confirmed that the detection wavelength band was broadened in the same manner as that schematically shown in the lower graph of FIG. This is because a regular arrangement structure of group III elements locally formed in the plane of the quantum well layer, in other words, by giving a distribution of plural kinds of compositions to the quantum well layer, the excitation level EL2 This is because of the variation of.

  Note that the broadening of the band of the infrared detector by the regular arrangement structure of group III elements has a great effect when this arrangement structure is provided in the quantum well layer as described in the present embodiment. However, this arrangement structure may be provided in the barrier layer.

  Also, the material of the quantum well layer and the barrier layer may be AlGaAs, InGaP, AlInAs, etc. in addition to InGaAs. Furthermore, the number of group III elements is not limited to two, but may be three or more, such as AlGaInAs. Furthermore, not only the group III element but also the group V element such as GaAsSb is similarly provided with a regular arrangement structure, that is, a distribution of a plurality of kinds of compositions, thereby widening the band of the infrared detector. Can do.

[Embodiment 3]
In the first and second embodiments, attention is paid to the excitation level EL2, and the detection wavelength band is broadened by giving variations in thickness or composition to the barrier layer or the quantum well layer to give variations in the excitation level. .

  The third embodiment relates to a technique for widening the detection wavelength band using variation in ground level energy.

  Since the detection wavelength of the infrared detector is determined by the energy difference between the excitation level and the ground level of the quantum dot electrons, it is possible to broaden the bandwidth not only by the variation of the excitation level but also by the variation of the ground level. is there.

  Variation in the ground level can be realized mainly by controlling the structure of the quantum dot layer. In the present embodiment, by varying the atomic layer unit intentionally to the heights of a plurality of quantum dots included in the quantum dot layer, the ground level EL1 varies as shown in FIG. It is characterized by making it.

  In the present embodiment, variations in atomic layer units are intentionally given to the height of the quantum dots. Usually, the size and shape of quantum dots vary by a certain width, which is called non-uniform spread. Due to this non-uniform spread, the ground levels of the quantum dots vary. This variation in the ground level is one of the factors that determine the detection wavelength band of the infrared detector.

  By the way, there is a method called an indium flash method as a method for reducing the variation in the height of the quantum dots. In the indium flash method, after a quantum dot crystal is grown, a buried layer as a partial cap layer having a predetermined height (thickness) lower than the quantum dot is grown, and the top of the quantum dot is formed from the partial cap layer. This is a method of evaporating the top of the quantum dot protruding from the partial cap layer by exposing a part of the surface and further performing a heat treatment. Usually, the growth of the partial cap layer is performed at a temperature similar to the growth temperature of the quantum dots, the thickness of the growth layer is about several nanometers, and the subsequent heat treatment is at or above the growth temperature of the quantum dots, preferably indium. Heating is performed to 500 ° C. or higher, which completely evaporates. As a result, the effective heights of the quantum dots are aligned. As a result, ground level variations are suppressed. That is, in the indium flash method process, according to the above process, quantum dots having a uniform height can be manufactured.

  On the other hand, in the present invention, in the indium flash method step, the growth of the partial cap layer and the heat treatment temperature are significantly lower than usual, thereby intentionally varying the atomic order in the quantum dot height. To grant.

  Specifically, quantum dots were formed as follows.

  First, according to the same process as the manufacturing method of the first embodiment, a buffer layer 2 (FIG. 1), a lower contact layer 3 (FIG. 1), and a semiconductor substrate 1 made of GaAs (FIG. 1) are formed by molecular beam epitaxy. A crystal growth process is performed up to the intermediate layer 10 (FIG. 2), the first barrier layer 11 (FIG. 2), and the first quantum well layer 12 (FIG. 2).

Further, a plurality of quantum dots 131 are formed on the first quantum well layer 12. The quantum dots 131 are self-formed by supplying InAs for 2.6 atomic layers in thickness at a substrate temperature of about 480 ° C. A typical quantum dot has a diameter of about 20 to 30 nm, a height of about 4 nm, and a number density per square centimeter of about 5 × 10 ¹⁰ .

  Thereafter, the substrate temperature is lowered to 420 ° C., and a buried layer 132 as a partial cap layer made of GaAs having a thickness of 2 nm is crystal-grown.

  Thereafter, the substrate temperature is increased to 480 ° C., which is the growth temperature of the quantum dots 131 (FIG. 2), and the second quantum well layer 14 (FIG. 2), the second barrier layer 15 (FIG. 2), By growing the intermediate layer 10 (FIG. 2) and repeating these steps, the light absorption layer 4 (FIG. 1) is manufactured. Thereafter, the infrared detector is manufactured in the same manner as in the first embodiment.

  In this embodiment, the growth temperature of the buried layer as the partial cap layer is lowered by 50 ° C. or more than the growth temperature of the quantum dots, and even if the heat treatment temperature is high, it is limited to the growth temperature of the quantum dots, We succeeded in varying the height of quantum dots on the atomic order. As a result, variation in ground level could be increased.

  FIG. 7A shows the photoluminescence emission characteristics from the quantum dot layer in the comparative example manufactured according to the normal process. On the other hand, FIG. 7B shows photoluminescence emission characteristics from the quantum dot layer 13 manufactured according to the present embodiment.

  By growing the quantum dots according to the process of the present embodiment, the half width of the photoluminescence emission, which is an index indicating the variation in the ground level of the quantum dots, is the quantum dot layer in the comparative example manufactured according to the normal process. Was about 30 to 40 meV (FIG. 7A), whereas it was about 70 to 80 meV (FIG. 7B). That is, the variation of the ground level of the quantum dot layer manufactured according to the present embodiment spread to about twice that of the comparative example.

  In the present invention, the interband transition energy indicating the variation in the ground level of electrons of each quantum dot is preferably 60 meV or more.

  By the way, when looking at the photoluminescence emission characteristics shown in FIG. 7B, the half-value width is about twice that of the comparative example (FIG. 7A). In (b), it can be seen that the light emission has a plurality of peaks indicated by arrows.

  The plurality of emission peaks are at intervals of about 30 meV, and this generally corresponds to emission from a group of quantum dots each including a plurality of quantum dots and having different heights in units of atomic layers. An example of such a quantum dot group is schematically shown in FIG.

The four types of quantum dot groups 131-1 to 131-4 schematically shown in FIG. 8 each include a plurality of quantum dots, and the heights (H _{Q1 to} H _Q4 ) are mutually in units of atomic layers. Are distributed in the plane of the quantum dot layer.

  The variation in the height of the quantum dots formed according to the usual method tends to be centered on a single height. In contrast, the quantum dots according to the present embodiment formed by using the indium flash method, as schematically shown in FIG. 8, have different quantum dot groups 131-1 to 131- having different heights in units of atomic layers. The distribution is 4. The emission wavelength of the quantum dots in such a quantum dot group mainly changes according to the difference in height (thickness). For this reason, as the number of types of quantum dots having a large distribution, that is, different heights increases, the ground level has a wider variation. As a result, there is an effect of widening the detection wavelength band of the infrared detector.

  The state of the electron ground level and the excitation level when the variation in the ground level of the quantum dots increases is schematically shown in FIG. That is, in the case of Embodiments 1 and 2, the infrared detection wavelength band is widened by the excitation level EL2 being varied, but in the case of the present embodiment, the ground level of the quantum dots is varied, and FIG. As shown in the lower graph of b), the infrared detection wavelength band becomes wider.

[Embodiment 4]
As in the third embodiment, this embodiment is characterized in that the ground level of the quantum dots is varied by making the structure of the quantum dot layer a special shape. For this purpose, the height of the quantum dots is varied by performing a short-time heat treatment immediately after the quantum dots are formed.

  The quantum dots formed in the embodiment of the present invention have a characteristic that they are easily diffused by heat. In particular, if the substrate temperature is raised to a temperature higher than the substrate temperature at the time of quantum dot growth immediately after crystal growth of the quantum dots, atoms constituting the quantum dots diffuse to the periphery, and the shape of the quantum dots is destroyed.

  Usually, quantum dots with a uniform shape including the height are required. Therefore, after the quantum dots are grown, the quantum dots are maintained for about several tens of seconds at the growth temperature of the quantum dots. The feedstock is fully diffused and incorporated into already formed quantum dots. Thereby, uniform quantum dots with uniform height are obtained.

  However, in the present embodiment, immediately after the crystal growth of the quantum dots 131 (FIG. 2), the substrate (FIG. 2) temperature is immediately raised slightly higher than that at the time of quantum dot growth, and the shape of the quantum dots 131 is intentionally collapsed. Variation is given to the height of the quantum dots 131. However, if the substrate temperature is raised too much, all the quantum dots 131 collapse. For this reason, the temperature to be raised is preferably within about 5 ° C., and the temperature raising time is preferably within about 10 seconds.

  After forming the quantum dot layer 13 (FIG. 2) including the plurality of quantum dots 131 formed with this heat treatment, the second quantum well layer 14 (FIG. 2), the second quantum well layer 14 is formed as in the first embodiment. The barrier layer 15 (FIG. 2), the intermediate layer 10 (FIG. 2) and the like are crystal-grown to complete a wafer for an infrared detector.

[Embodiment 5]
The present embodiment is characterized in that the quantum dot layer has a special shape so that the ground level of the quantum dots varies as in the third and fourth embodiments. For this purpose, the quantum dot height variation is created by subjecting the wafer for infrared detectors to rapid thermal processing.

  Usually, in the case of InAs, which is a material used for quantum dots, when rapid heating, that is, heating to a temperature higher than the crystal growth temperature of the quantum dots in a short time, the group III element In is inherent in the quantum dots. The interface between the quantum dot layer and the layer in contact with it diffuses from the position to the periphery, and the confinement efficiency is reduced. As a result, the variation in ground level is increased as compared with the case where rapid thermal processing is not performed.

  Specifically, a semiconductor wafer for an infrared detector is manufactured according to the following process.

  First, according to the same process as the manufacturing method of the first embodiment, a buffer layer 2 (FIG. 1), a lower contact layer 3 (FIG. 1), and a semiconductor substrate 1 made of GaAs (FIG. 1) are formed by molecular beam epitaxy. A crystal growth process is performed up to the intermediate layer 10 (FIG. 2), the first barrier layer 11 (FIG. 2), and the first quantum well layer 12 (FIG. 2), and further, a quantum dot layer 13 made of InAs (FIG. 2). Are laminated.

The quantum dot layer 13 is self-formed by supplying 2.6 AsA of InAs at a substrate temperature of about 480 ° C. A typical quantum dot 131 (FIG. 2) has a diameter of about 20 to 30 nm, a height of about 4 nm, and a number density per square centimeter of about 5 × 10 ¹⁰ .

  After crystal growth of the quantum dots 131, the substrate temperature was immediately increased by 3 ° C. and held for 10 seconds as in the fourth embodiment. By this step, a large variation can be imparted to the height of the quantum dots.

  After forming the quantum dot layer 13 (FIG. 2) including the plurality of quantum dots 131 formed with this heat treatment, the second quantum well layer 14 (FIG. 2), the second quantum well layer 14 is formed as in the first embodiment. The barrier layer 15 (FIG. 2), the intermediate layer 10 (FIG. 2) and the like are crystal-grown to complete a wafer for an infrared detector.

  Next, rapid heat treatment is performed. The wafer for the infrared detector after crystal growth is introduced into a chamber filled with nitrogen, rapidly heated by an infrared heating lamp, and then rapidly cooled.

  The heating temperature is preferably 600 ° C. or higher, which is the highest temperature at the time of crystal growth. The upper limit temperature is preferably up to a temperature at which As which is a constituent group V element does not desorb from the crystal site by heating. Actually, heating up to about 800 ° C. is possible for a short time.

  A sample (this embodiment) which is an infrared detector manufactured with rapid thermal processing of a wafer and a sample (comparative example) which is an infrared detector manufactured without rapid thermal processing of a wafer are manufactured. The detection characteristics were measured. FIG. 9 schematically shows the measurement results.

  FIG. 9A shows the characteristics of a sample not subjected to rapid thermal processing (comparative example), FIG. 9B shows the characteristics of a sample subjected to rapid thermal processing at 800 ° C. for 2 minutes (this embodiment), FIG. 9C shows the characteristics of a sample (this embodiment) subjected to a rapid heat treatment at 800 ° C. for 4 minutes.

  From FIGS. 9A to 9C, it can be seen as a sensitivity characteristic that there is a light responsiveness as an infrared detector over a wavelength range (Δλ) around the maximum value of the light responsiveness. That is, a comparative example in which rapid thermal processing is not performed (FIG. 9A), this embodiment in which rapid thermal processing is performed at 800 ° C. for 2 minutes (FIG. 9B), and rapid thermal processing is performed at 800 ° C. for 4 minutes. In this order of the present embodiment (FIG. 9C), it was confirmed that the infrared detection sensitivity band was widened.

[Modification]
As mentioned above, although Embodiment 1-5 of this invention was demonstrated concretely, this invention is not limited to these embodiment, A various deformation | transformation is possible if it is in the range of the technical idea of this invention. It is.

  For example, the methods described in each of the first to fifth embodiments are effective even when used alone, but a greater effect can be achieved by combining a plurality of methods. That is, by combining at least one of Embodiments 1 and 2 and at least one of Embodiments 3 to 5, as shown in FIG. 10A, the ground level EL1 and the excitation level EL2 of the quantum dots are obtained. Both variations can be enlarged. As a result, as shown in the lower graph of FIG. 10B, the detection wavelength band is significantly widened as compared to the normal infrared detector as a comparative example shown in the upper graph of FIG. Is feasible.

  A part or all of the above-described embodiment can be described as in the following supplementary notes, but is not limited thereto.

(Appendix 1) A quantum dot layer including a plurality of quantum dots and the quantum dot layer sandwiched between the pair of quantum well layers having a larger band gap energy than the quantum dot layer and the pair of quantum well layers, A pair of barrier layers having a band gap energy larger than that of the pair of quantum well layers, and a pair of intermediate layers sandwiching the pair of barrier layers, and a base in the quantum dots of electrons based on excitation light as detection target light An infrared detector that utilizes a transition between a level and an excitation level,
At least one of the quantum well layer, the barrier layer, and the quantum dot layer is provided with variation in at least one of its structure and composition,
Accordingly, at least one of the excitation level and the ground level of the electrons of each quantum dot has a size variation corresponding to a desired detection bandwidth.

(Appendix 2) At least one of the quantum well layer and the barrier layer,
The average thickness is ¼ or less of the height of the quantum dots,
The thickness variation is greater than or equal to a predetermined size, and
At least one of spatially varying constituent element compositions is provided,
Accordingly, the infrared detector according to appendix 1, wherein the excitation level of each electron of the quantum dots has a variation in magnitude according to a desired detection bandwidth.

(Supplementary Note 3) At least one of the barrier layer and the quantum well layer,
The average thickness is 1 nm or less, or
It is given that the average thickness variation is 4 atomic layers or more,
Accordingly, the infrared detector according to appendix 1, wherein the excitation level of each electron of the quantum dots has a variation in magnitude according to a desired detection bandwidth.

(Appendix 4) At least one of the quantum well layer and the barrier layer is provided with a plurality of types of composition distributions in the plane,
Accordingly, the infrared detector according to appendix 1, wherein the excitation level of each electron of the quantum dots has a variation in magnitude according to a desired detection bandwidth.

  (Appendix 5) The composition distribution imparted to at least one of the quantum well layer and the barrier layer is such that a plurality of types of group III elements formed in at least one surface of the quantum well layer and the barrier layer are regular. The infrared detector according to appendix 4, characterized in that the infrared detector is constituted by a structure arranged in the above.

(Appendix 6) The plurality of quantum dots included in the quantum dot layer are provided with a variation in height of a predetermined size or more,
2. The infrared detector according to appendix 1, wherein the electron ground level of each quantum dot has a variation in magnitude according to a desired detection bandwidth.

  (Supplementary note 7) The variation in height given to the plurality of quantum dots includes a plurality of quantum dots, each of which includes a plurality of quantum dots, and is configured by a distribution of a plurality of quantum dots having different heights in units of atomic layers. The infrared detector according to appendix 6, which is characterized.

  (Additional remark 8) The infrared detector in any one of additional remark 6 thru | or 7 whose interband transition energy which shows the dispersion | variation in the ground level of each quantum dot is 60 meV or more.

(Supplementary note 9) The quantum dots are made of InAs or InGaAs,
The quantum well layer is made of InGaAs or GaAs,
9. The infrared detector according to any one of appendices 1 to 8, wherein the barrier layer is made of AlAs or AlGaAs.

(Supplementary Note 10) A step of forming a quantum dot layer including a plurality of quantum dots, a step of sandwiching the quantum dot layer and forming a pair of quantum well layers having a larger band gap energy than the quantum dot layer, and the pair A pair of barrier layers having a band gap energy larger than that of the pair of quantum well layers, and a step of forming a pair of intermediate layers sandwiching the pair of barrier layers, A method of manufacturing an infrared detector using a transition between a ground level and an excitation level in the quantum dot of electrons based on excitation light that is detection target light,
At least one of the quantum well layer, the barrier layer, and the quantum dot layer is given a variation in at least one of its structure and composition, whereby the excitation level and the ground level of each electron of the quantum dot A method of manufacturing an infrared detector, characterized in that at least one of them has a variation in size according to a desired detection bandwidth.

  (Additional remark 11) The infrared detector of Additional remark 7 characterized by the photoluminescence light emission from the said quantum dot layer showing the multipeak light emission resulting from the height of each of these quantum dot groups.

  INDUSTRIAL APPLICABILITY The present invention can be used for an infrared detector used for environmental measurement such as heat detection and gas detection in a medium wavelength infrared region and a long wavelength infrared region (approximately 3 to 14 μm).

DESCRIPTION OF SYMBOLS 1 Semiconductor substrate 2 Buffer layer 3 Lower contact layer 4 Light absorption layer 5 Upper contact layer 6 Upper electrode 7 Lower electrode 10 Intermediate layer 11 First barrier layer 12 First quantum well layer 13 Quantum dot layer 131 Quantum dot 132 Embedding Layers 131-1 to 131-4 Quantum dot group 14 Second quantum well layer 15 Second barrier layer 16 Electron X excitation light (detection target light)

Claims (10)

A quantum dot layer including a plurality of quantum dots; and a pair of quantum well layers sandwiching the quantum dot layer and having a larger band gap energy than the quantum dot layer; a pair of quantum well layers sandwiching the pair of quantum well layers; A pair of barrier layers having a larger band gap energy than the layer, and a pair of intermediate layers sandwiching the pair of barrier layers, and the ground level and excitation in the quantum dots of electrons based on excitation light as detection target light An infrared detector that utilizes a transition between levels,
At least one of the quantum well layer, the barrier layer, and the quantum dot layer is provided with variation in at least one of its structure and composition,
Accordingly, at least one of the excitation level and the ground level of the electrons of each quantum dot has a size variation corresponding to a desired detection bandwidth. At least one of the quantum well layer and the barrier layer,
The average thickness is ¼ or less of the height of the quantum dots,
The thickness variation is greater than or equal to a predetermined size, and
At least one of spatially varying constituent element compositions is provided,
The infrared detector according to claim 1, wherein the excitation level of each of the quantum dots has a variation in magnitude according to a desired detection bandwidth. At least one of the barrier layer and the quantum well layer,
The average thickness is 1 nm or less, or
It is given that the average thickness variation is 4 atomic layers or more,
The infrared detector according to claim 1, wherein the excitation level of each of the quantum dots has a variation in magnitude according to a desired detection bandwidth. At least one of the quantum well layer and the barrier layer is provided with a distribution of a plurality of types of compositions in the plane,
The infrared detector according to claim 1, wherein the excitation level of each of the quantum dots has a variation in magnitude according to a desired detection bandwidth.   The composition distribution imparted to at least one of the quantum well layer and the barrier layer has a structure in which a plurality of types of group III elements formed in at least one plane of the quantum well layer and the barrier layer are regularly arranged. The infrared detector according to claim 4, comprising: The plurality of quantum dots included in the quantum dot layer are given a variation in height of a predetermined size or more,
The infrared detector according to claim 1, wherein the ground level of the electrons of each quantum dot has a variation in size according to a desired detection bandwidth.   The variation in height imparted to the plurality of quantum dots includes a plurality of quantum dots, each of which includes a plurality of quantum dot groups having different heights in units of atomic layers. Item 7. The infrared detector according to Item 6.   The infrared detector according to any one of claims 6 to 7, wherein an interband transition energy indicating a variation in an electron ground level of each of the quantum dots is 60 meV or more. The quantum dot is made of InAs or InGaAs,
The quantum well layer is made of InGaAs or GaAs,
The infrared detector according to claim 1, wherein the barrier layer is made of AlAs or AlGaAs. A step of forming a quantum dot layer including a plurality of quantum dots; a step of sandwiching the quantum dot layer and forming a pair of quantum well layers having a band gap energy larger than that of the quantum dot layer; and the pair of quantum well layers A pair of barrier layers having a band gap energy larger than that of the pair of quantum well layers, and a step of forming a pair of intermediate layers sandwiching the pair of barrier layers. A method of manufacturing an infrared detector using a transition between a ground level and an excitation level in the quantum dot of electrons based on a certain excitation light,
At least one of the quantum well layer, the barrier layer, and the quantum dot layer is given a variation in at least one of its structure and composition, whereby the excitation level and the ground level of each electron of the quantum dot A method of manufacturing an infrared detector, characterized in that at least one of them has a variation in size according to a desired detection bandwidth.

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